Apart from the obvious fact that sessions aren't dances*, the reel point is cooperation. A session is a conversation between players, and in any conversation it's simply bad manners not to accommodate the other person's interests as far as you can. Hence where there's a reasonable choice of venues (getting rare these\days), they tend to crystallise out according to the players. The high- power players don't go to the slower sessions, and the beginners soon learn their mistake if they choose the supersession. The majority of us drift inbetween, playing as well as we can with the big boys, and enjoying playing with and even teaching those who struggle a bit.

* Dancers have turned up to some sessions I've been to, and insisted that the musicians play the way they want for dancing. They are usually as welcome as a didgeridoo player with an empty wallet.