I don't think I've ever noticed a habit of leaving after doing their spot. Most people, in places I've been tend to stay to the end. I have in the past witnessed related issues though.

Some seem to like to arrive in time for their (perhaps first) spot. Maybe you find there are never many floor singers around when the club stars at 8:30 as they can't make that time. After discussion with all, you decide to start at 9:00pm. The people who couldn't make 8:30 then can't make 9:00.

You may find those who have been away for a few weeks have not realised it's a guest night the time they all happen to by co-incidence turn up on the same night armed with guitars expecting a spot. (And yes you may also observe that some of these are the ones who had ceased to be able to make the later start time...)

Maybe it's just me, but while there may in my experience be pretty good manners overall, I'm not sure I've always witnessed much in the way of interest in trying to support a night and pulling together for the benifit of all.