I can only relate my experience. I play in Steve Creech's game at a local game store. Steve runs (at least) three games for them, every other Saturday. A beginner, an intermediate and an advanced game.
We play at a set time, though players seem to come and go. There are two of us who have been with it since the beginning, and another who started with our third or fourth session, I think, who seems to have made a long term committment to it. The rest of the folks might show up for a session or two, and then move on. Or maybe not. You never know when one of the n00bs will decide to stick around. Last session we had two new players. Will they show up next Saturday? We shall see.
I'd imagine it would be difficult for a DM to deal with all the comings and goings--I know it would be for me. But Steve seems to handle it with no trouble.