A LOT of seperately managed account managers do not run or subadvise mutual funds. So the general retail public does not have access to them. They do not want to deal with the regulations and marketing that go along with mutual funds (like indepenedent boards).
Find Aletheia from LA. They run large cap growth and value. Excellent returns in both up and down markets. Do they run a mutual fund?...no. The only way you can get access is through a seperate account. I can name off a dozen of these managers.
Certain managers are excellent in their particular asset class, but are marginal in others. Do you really think that Fido or American has a full line up of rock stars in each asset class? Again, the answer is NO. Instead, you can construct a portfolio of managers who are specialists in their particular asset class. For example, a small cap value manager can be great in that space. But if you asked him to pick large growth stocks, he (or she) would suck. It isnt their speciality.
If you ran the numbers looking at only seperate account managers, you'd see what I am talking about.
As far as diversifying your alpha source from different managers...I am not saying put a dozen large cap managers in your portfolio for your large cap allocation. I agree you'd revert to the mean over time. I am saying you find the best one or two managers that specialize in the space. You also balance out the aggressivness between the managers.
Tax loss selling....The manager runs a 50 stock portfolio. At any given time, some positions have gains and some may have losses. With a seperate account, you go into the portfolio and sell those with losses. i.e. You own KO that is at a loss. The PM still likes the name for whatever reason (emerging market growth opportunities). You sell KO and recognize the loss. Individuals can use capital losses to offset capital gain. Capital losses can be carried forward forever if they cannot be used today. The PM uses the cash raised to buy PEP. PEP and KO have a high correlation and are impacted by the same macro trends. You wait 31 days to avoid wash sale rules (to avoid draging losses). You sell PEP and repurchase KO. Assuming the two stocks didnt move, you have just realized a capital loss that can be used to offset capital gains. It should be obvious that this strategy does not work for tax exempt entities.