Even if you're playing with nice folks and all that, there are plenty of stories of folks who just didn't click at the gaming table.
I've had friends that I otherwise got along with pretty darn well (in the past) but who's gaming style was too different from mine for us to enjoy playing together much, I think. I was just annoyed at them all the time. Specifically I'm thinking of a player who'd get bored if there was too much "roleplaying" going on and then just jump in and attack something just to have something to do.
But I wouldn't have drawn up a contract, I don't think. I also wouldn't have kicked her out, because we were otherwise friends. As it turned out, her and her husband moved away, and as they were a good 40% of our gaming group, between the two of them, I ended up having to completely restructure.
But I guess the rambling point here is that just because your friends are, well, your friends, and are decent people and all that, doesn't necessarily mean you'll enjoy playing with them.