#3 You get a feel for the general style of play on the network (each site seems to be a little unique) as well as a feel for the MTT structures which really helps for setting up optimal counter strategier.
I agree a lot with this point, and it's interesting to hear from a pro, because I feel it is rarely mentioned. I think there are clear differences in playing styles and player tendencies between different sites and networks, also in cash games. A strategy/style that works perfectly on one site does not necessarily work equally well on another site - at least that is my experience (and I have the losses to back it up ;p).
That said, to answer the original question: I have accounts on a lot of sites but currently keep bankroll on four, and am playing actively on three at the moment. I choose these because of one or more of the following:
* Softness of competition
* Selection of different game variants
* Quality of software and general reliability
Since I live in a regulated market that does not allow rewards anymore, rakeback and promos are a non-factor for me.