I play on Full Tilt for my rakeback, and maybe 20% on Pokerstars for a change of scenery and game (love the mental NL omaha SnG's).
Used to play Laddies but my comp wont run it at full speed - plus I used to donk off most of my winnings in the casino bit!
Do they still have the 'double end' SnG's where first and last place both cash?
The benefits of playing loads of sites are
A)Opening a real-money account can open freerolls for depositing players only
B) You get a first-deposit bonus to clear and
C) you will get offers of free cash, free unlockable bonuses e.mailed from various sites all the time. i.e. FT poker may run a free $100 to unlock in June, then in July it might be Stars, so you can switch them up fir the best bonus.
It is better to concentrate on one or two sites though, simply to build up the frequent player points. They don't often amount to much, but better than nothing.