@hank - I'm always happy to admit I've overlooked something. (Well, not exactly happy, but willing :-)) So I went back and checked the holdings (1Q2016) of TRRIX.
None of the funds it holds are marked as institutional. In contrast, the holdings of TRPTX are all listed as
<fund name>-I, indicating institutional class.
As you mentioned, TRP's institutional class funds are open to retail investors; for TRPTX all you need is a million bucks. Though they are more welcoming if you buy the shares through a retirement plan at work.
TRRIX holdings: http://individual.troweprice.com/staticFiles/gcFiles/pdf/phrpeq1.pdf
TRPTX holdings: http://individual.troweprice.com/staticFiles/gcFiles/pdf/phrqiq1.pdf

One PSLDX minim appears to be $100k while M* says $1M. Other than that...

@msf - You and I can't buy their institutional shares of some of the underlying funds - which do carry lower costs (unless you've got whole lot more money than I think). :) I'll rest my case there.
However, I read somewhere (over a decade ago) that Price does actually give back a small portion of the underlying ERs to their allocation funds - on the theory that they save money in the long run by not having as many separate accounts to manage. Put another way, they'd rather have you put all your money into RPSIX than have you own smaller amounts of 12 different income funds. It made sense to me at the time, but unfortunately, I've never been able to verify it. (And I've searched their Prospectus's trying to verify)
You are completely correct on the second point. While TRRIX is classified a retirement fund by Price, alongside their target date funds, it is actually an asset allocation fund. I guess you could say that I avoid the target date variety of retirement funds (as most everyone here is probably inclined to do).
Edmond: No intent on my part to in any way promote Price, its funds, actively managed funds, any particular investing style, or allocation and target date funds. To each his own. What caught my eye was the lower than expected "average" fund expenses stated in Ted's linked article. In trying to make sense of those numbers I strayed too far afield. But you make some excellent points.

That's an interesting webpage, one I've visited in the past. But casual viewers should use those Vanguard-collected status with care.
Consider:
The span of those 'average returns' is 89 years. "Averaging returns" is a mathematical exercise, but investors don't experience smoothed-over 'averaged' returns. They experience sequential, erratic returns. Certainly, 20- or 30-somethings might look at those average returns and reasonably conclude to go "all-in" to equities. -- But it may not make a lot of difference for a typical young investor --- as they have relatively little saved. Most or all of income often going for raising kids, placing a down-payment on a home, student-debt servicing, kids' college or for some, 'living large'. By the time many households get round to saving serious dough (many never do!) they may be in their 40's or 50's. Are average 89-year returns something they should expect? What about when they begin the distribution-phase and are withdrawing assets (via RMDs, etc). Should they count on historical 'averaged' returns, or make some reasonable (and conservative) estimate of future returns based on asset prices? I believe John Bogle and others do offer those estimates in interviews from time to time, based on today's prices as a "set-up" for likely prospective returns. Buying long-term Treasurys in 1982 would have reasonably generated a certain forward-return over 30 years. Buying long-term Treasurys today, prospective-returns will be much more compressed.
Even if an investor could obtain a GUARANTEE of receiving those average returns (say in the form of an insurance company annuity etc.) at the terminal date (89 years), if they had to wait 89 years to receive that payout, would they be alive to collect it? My point: 89-year average returns, even if 'guaranteed' are not meaningful for individuals, if one is pushing up daisies when the guarantee is due to them. Ask Japanese equity investors who bought/held in 1988 and are still well under-water today, 30 years later. -- Hey in another 59 years they may enjoy the fruits of their patience....
Then too, the Vanguard site lists the historical return on a 100% bonds portfolio as 5.4%. Consider AGG, which is a proxy for the total (non-junk) US bond market. Presently, the SEC yield on AGG is 1.72%, with an average coupon of 3.2% The average-price of the bonds trade $9 over par. Perhaps 89 years from today, average-returns may match that. Nobody reading this will be around then. What are the likely returns over the next 1,3, 5, and 10 years? --- This would seem to be more relevancy. Are forward returns, using today as a starting point, likely to be closer to 89-year historical, averaged returns OR the SEC yield?

@MFO: In my opinion, when someone is considering their fund allocation to bonds/equities or both combined, I suggest they review Vanguard Portfolio Allocation Models from 1926-2015 that show the historical rate/risk returns from 100% stocks to 100% bonds and the various combinations of both.
Regards,
Ted
https://personal.vanguard.com/us/insights/saving-investing/model-portfolio-allocations

"The average charge for a target-date fund is 0.73% ... The average stock fund charges 0.68%, while the average bond fund charges 0.54%."
I'm surprised the averages are that low. Perhaps if index funds and ETFs are included that's true.

What likely sounds more familiar to you are figures around 1.25%. That's what one gets by equal-weighting funds - giving as much weight to AMDEX (ER 3.38%, an index fund tracking the Israeli market) as to VFINX.
Here's a M* paper from 2014 giving weighted and unweighted (i.e. equal-weighted) averages of funds from 1990 through 2013 - overall averages and averages by fund type. The 2013 figures are very close to the current ones (e.g. 0.67% for weighted average of all US equity mutual funds). You're right that these figures include index funds, but they exclude ETFs, as those are not open end funds.

Contrary to industry practice, you'll find Price's asset allocation and target date funds generally a bit cheaper to own than if you bought the underlying funds yourself.

Theoretically, a fund can't be cheaper than buying the underlying funds. The TRP fund "will indirectly bear its pro-rata share of the expenses of the underlying T. Rowe Price funds in which it invests (acquired funds)." (from prospectus).
Most families may add fees on top of what it costs to own the underlying funds. But there's more going on here. Look at Fidelity - it tacks on a modest fee (about 8 basis points) to its Freedom Index target date funds. The underlying funds are cheap index funds with little profit margin to pay for the extra overhead of running target date funds. In contrast, Fidelity Freedom funds (that invest in actively managed funds) don't tack on an additional fee.
T. Rowe Price may be using index funds as some of the underlying funds, but they're known to be not particularly cheap. So TRP doesn't need to tack on additional fees.
Vanguard takes a different approach. Its target date funds invest in higher cost Investor class shares, rather than Admiral shares. That difference allows it to absorb the target funds' administrative costs without losing money.

The reason I prefer TRRIX to their other retirement funds is that this one doesn't follow any glide slope. (A bit of a control freak).

What it sounds like you want is an asset allocation fund, as opposed to a target date/retirement fund. TRP appears to call TRRIX a retirement fund (as opposed to a Personal Strategy, i.e. asset allocation fund), only because it is a fund of funds; not because it is a real retirement fund.
There are only a few parameters that differentiate various groups of hybrid funds:
1) Glide path yes/no (target date/asset allocation)
2) Fund of funds yes/no (minimal extra overhead for fund of funds, sometimes absorbed)
3) How aggressive (for target date funds, the glide path; for asset allocation the "neutral" mix)
4) Indexed (primarily for funds of funds - are the underlying funds index funds)
BTW, I applaud TRP for offering two different classes of funds with different glide paths, so that one can choose the level of aggressiveness.

A couple things:
I think the TERM 'multi-asset fund' (with or without income) is a creation of fund-industry marketing types.
Any old-fashioned balanced fund which emphasizes income is a 'multi-asset income fund'. (MAIF) I own one, its called Vanguad Wellesley.
Some newer MAIFs add in riskier sleeves-- junk, MLPs, whole-loan products, etc. etc. They are simply stepping out on the risk-spectrum. --- the higher the yield (i.e. the bigger the spread), the bigger the risk.
While I still own some VWINX, my thinking on hybrids (MAIFs or otherwise) has evolved over the past 3-5 years. I generally eschew them, in favor of single-asset products. I suspect that 'bundling' assets has the effect of obscuring how good the manager of each asset-sleeve within the 'bundle' is. [Take OAKBX, for example. The stocking picking was always very good, but Oakmark never had much of a bond desk. They usually loaded their bond-sleeve with Treasurys (US & Canadian) and that was it. The stock picking "carried" the fund overall for many years. Til it didn't.] How does an individual investor get comfortable that each of the asset-sleeves within a hybrid (or "MAIF) is at least average -- and hopefully above so?)
Then too, every asset class will encounter an investing environment with a lousy "setup". At those times, its best to UNDERweight such assets. Most hybrid products have allocation guidelines which require weightings stay within a certain range. Having witnessed 3 massive boom-bust cycles this century (tech, mortgage, energy), I am uncomfortable with "forced" allocations. The lower rates go, the more convinced I am, we are in the 'pleasant' phase of a 4th boom-bust cycle.
Another thing: all commingled products, MAIFs included, assess a MER equally, across all AUM in the fund. But is it really serving investors best to levy (for example only) a flat 1.25% for managing the equity-sleeve, and the same 1.25% for the bond-sleeve. -- Especially if that bond sleeve is often Treasurys, Agencies, and investment-grade paper. --- And especially now that bond coupons barely will cover that MER....). I mean you can find superlative dedicated-bond managers for ~0.50%. Paying 1.25% for the bond sleeve of a hybrid fund seems like you are paying for that bond manager's Alfa Romeo...
FPACX is a good example. It has a superior performance, granted (though performance seems to have suffered with growing assets). It levies a 1.09% on AUM. 34% if AUM is sitting cash. Institutional MMFs are paying NOTHING! I'm not arguing Romick isn't prudent in holding cash --- rather that he should not be charging investors 1.09% for the cash-sleeve. A hybrid can get away with this. Dedicated-asset funds are less prone to charging investors for NOT investing their money.
I'd much prefer to choose single-asset "best of breed" managers for the major asset classes myself. In some cases (say L/C US equities) the "manager" may be Standard & Poors (i.e the index). In others (foreign equities, S/C, REITs, bonds), active managers who persistently excel may be able to be identified. As an asset-class "smells bubbly", I can trim back, rather than relying on a professional allocator, who is frankly, not at all concerned if Edmund's nest-egg is halved due to a forced allocation in a hybrid product.
Just my opinion.

FYI: It is easy to confuse day-to-day noise with actual and significant signals. If you are merely reacting to the latest market action, then what you have is not a plan — you have an instinctual, fear-driven reaction, and it’s the makings of a disaster.
Regards,
Ted
https://www.washingtonpost.com/business/get-there/given-the-brexit-brouhaha-how-did-your-investments-hold-up/2016/07/22/a7bc1198-4d03-11e6-a7d8-13d06b37f256_story.html

"The average charge for a target-date fund is 0.73% ... The average stock fund charges 0.68%, while the average bond fund charges 0.54%."
I'm surprised the averages are that low. Perhaps if index funds and ETFs are included that's true. But for actively managed funds those .68% and .54% averages look low to me. A good actively managed international stock fund often exceeds 1% in expenses.
The debate over target date funds (which vary widely) has been going on now for at least a decade. My thinking is that people who don't read MFO or otherwise take much interest in investing are probably well served by them. Think of them as a default option. For the more financially inclined/literate there's better approaches.
The only retirement fund I own is TRRIX, a conservative 40/60 fund with a .56% ER. Price keeps cost down in part by investing 20% in their S&P index fund. You also get 14-15% exposure to some of their international equity funds. Contrary to industry practice, you'll find Price's asset allocation and target date funds generally a bit cheaper to own than if you bought the underlying funds yourself.
The reason I prefer TRRIX to their other retirement funds is that this one doesn't follow any glide slope. (A bit of a control freak).

In response an the original question: do multi-asset income funds further diversify a portfolio or are they a "fad", my thinking is no and no.
Until the 1990s, funds tended to be classified and analyzed according to their objective, e.g. aggressive growth, growth, G&I, equity income. Along came style boxes followed by an emphasis on style purity. Different questions (what vs. how).
A fund managed for objective (very fuzzy, which is why M* rates on style) will likely cut across many styles. But that doesn't make your portfolio more diversified than one where you hit the same holdings by creating your own mix of funds with different styles.
IMHO there's been a gradual shift back toward funds focused on objectives (like income) rather than style - with target date funds for example. Income-oriented funds seem like another natural. Just as the old growth, G&I, etc. categories represented a dialing up or down of risk (and income vs. appreciation) in the equity arena, income investing seems ripe for a similar sort of grouping by objective.
In part, that's because there are more vehicles than ever for generating income - well beyond the investment grade/junk/muni triumvirate of a generation ago. In part, that's because interest rates have been so low for so long that there's demand for other ways of generating income without going off the risk charts. In part because boomers are retiring and interested in generating income. That's what they need regardless of which of their funds hold what types of bonds.
So, no these funds don't add diversification; though they may facilitate portfolio construction. And no, they're not a fad, if for no other reason than there are lots of more esoteric vehicles that are better employed as parts of other funds than as standalone products.

FYI: One of the biggest challenges for gold as an investment is that, like cash, it doesn't really generate income.
When there are bonds and stocks out there that offer interest and dividend yields to offset the risk of any fall in capital values, why choose a shiny metal that's a pure punt on price movements?
Regards,
Ted
http://www.bloomberg.com/gadfly/articles/2016-07-22/gold-s-9-2-trillion-tailwind

It is a regional hospital group, so yes, they pay their staff quite well! They took this action after analyzing their employees 401k accounts and wanting to insure they had good retirement savings. This may be a shock to some on this board, but some employers care about their employees!

@Willmatt72: My own multi-asset bond fund is a global one: PRSNX. Very un-volatile, but extremely pleasing to me re: performance. I bought in Feb. of 2015. Over 17 months, it's given me +4.55%. It's in a Trad. IRA.

Gas here now obtainable at $2.04--- a reflection of the cost of oil. I have an infinitesimally small position in COP. Doing nothing for me. I continue to hold it, noting how ratings of the stock's estimated high put it at anywhere between $44 (JPM) to $71 (BofA-ML). The stock is just above $40 right now. It won't kill me to be patient... Meanwhile, back at the ranch, ("it's a song about Alice, remember!") I hold no dedicated HY bond fund. My all-bond funds are: DLFNX (core-plus), PRSNX (Global multi-asset), and PREMX (EM bonds.) Let the fund Managers do their job, eh? That listing is in order, smallest to highest proportion right now in the portfolio: 2.48%, 10.87% and 14.51%. Very pleased with the solidity and low volatility of DLFNX. PRSNX became a holding on a very tardy basis, recalling a recommendation from someone on this discussion board back in 2009 or so. I've held PREMX since 2010, and it grew to be a riskier than I thought size. I've since reallocated a great deal of it. Gotta be pleased with it THIS year!

My wifes company is requiring their employees to up their contribution rate to 7%(from 6) this year, 8% next year, 9% the following year, then freezing at 10% the next! I think this is a good thing.

I believe the 7 day yield is neither APR nor APY, but a hybrid - the compounded yield over a seven day period, extrapolated as simple interest over a year. Not that it makes any difference at these low rates (as you'll see below), but here's the complete calculation:
0.53% 7 day SEC yield equates to 7/366 * 0.53% ~= 0.010137%
Remember that this is an election year (how could one forget?), so 366 days.
That 0.10137% is a the total return over a week, i.e. a compounded daily return.
So the daily rate is (1 + 0.010137%) ^ (1/7) - 1 ~= 0.001448%.
On $1M, that yields $14.48 per day.
The deviation from Ted's figure is virtually all due to the leap year. Redoing the calculation above with a 365 day year produces $14.519, vs. Ted's $14.521. A daily difference of just two mill.
All this is academic, not because I don't seem to have $1M in pocket change, nor because the minimum investment is $100M, but because BGIXX (Black Rock Cash Funds Institutional, Institutional Class) is closed. See summary prospectus for this share class or its SAI.

@MFO Members: The patient died caused by small AUM, high ER, and the following performance percentile of YTD 96, 1Yr. 96, 3Yr. 95, 5yr. 98 !
Regards,
Ted

@MFO Members: Using BlackRock Cash Funds Institutional 7-Day yield 0.53
One has $1,000,000 invested for 30 days at a 7-day SEC yield of 0.53 then:
(0.53 × $1,000,000 ) / 365 ~= $14.52 per day.

The thirst for yield continues unabated. Today is something like the 11th day this month junk bonds have hit all time highs. About 2 weeks ago for the first time in 18 months junk bonds began ignoring oil. Curious if that will continue if oil breaks below 40. In the meantime all you can do is go with the flow.

Currently, Fidelity's retail MMF FZDXX ($10K min in IRA) is yielding 0.45%, while the Fidelity institutional MMF listed by iMoneyNet, FIDXX, is yielding 0.44%.
These figures are as of June 30th, 2016 according to the linked pages. Price stability (no floating NAV) and higher yield. Sometimes the retail investor comes out better.
(Note that this is happening because Fidelity is waiving more of the retail fund's fees than the institutional fund's fees. Without waivers, the institutional fund would be ahead, 0.41% vs. 0.37%.)