I suppose for some, one room is enough. And, for others, maybe more than one. I guess for each individual there are a lot of reasons for either choice. They're all valid reasons.
I play
poker stars and 888. I would probably play at others if I knew of any with similar quality. My first choice is always Poker Stars for too many reasons to go into. My second is 888. I really don't know any other rooms that offer what I am looking for. (I am open to suggestions.)
Why I like to play at more than one room goes simply to my schedule. Often, there are no free rolls available when I have the time to play. So, being able to go to more than one room, or to more than two for that matter, is really a matter of convenience.
I can only play when I have the time to play. So, when one room has no free rolls, I travel to the next. It would be nice if I knew a few more rooms in which to play Holdem, but I don't, so there it is.
So, for myself anyways, it just makes sense to play in a lot of poker rooms. (Not that two is really a lot.) It just gives me the ability to play free rolls according to my personal schedule.