I was always wondering why would someone do that.....
1. It is more than certain that, whatever you'll be mining, you will get less coins for your hashrate. All rigs have smaller hashrate on multipools and get a higher number of stales, due to the speed they change coins. The higher the speed they change coins, the worse your rig will perform.
2. All multipools fail to follow difficulty and price changes, when they start mining a coin, it is usually time to move on another one.
3. Multipools can't mine the most profitable coin, because of the algorithms they use. When X coin has a temporary drop in price, it is excluded from "profitable coins" automatically... It's even worse when you let them exchange for you, when the coins get to the exchange, they never have the value you were mining them for, because several hours have passed (if not days).
4. When you let them exchange the coins for you, you always get screwed, they don't get the highest price for what you mined or the lowest price for the coin you want and you pay extra fees.

I'm mining for a very long time, sha256 and scrypt algos, I never use multipools, no matter the reason, prefer to do the maths myself.

Edit: and I didn't even touched the subject "transparency"..... there is no such thing in multipools, you never know whats going on.