There has been a bit of divergence in this thread and floor singers/spots have featured heavily in the discussion.

Yes, the OP is referring to "booked acts" but he or she also states

"Mind you, these are just 20 minute sets, which means that anyone can sit through practically any one of them."

I realise that all clubs are organised differently but, in certain quarters, a 15-20 minute set could also be an extended floor spot.

Certainly, if the artists have been advertised to play at the venue and the punters have bought tickets on the strength of this then the organiser has an extra responsibility to the audience.

However, if the 15-20 minutes set is just a "warm up" for the main act who may be doing something like two 45 minute sets, then this may be seen as less crucial. Of course, as I said earlier, it's not necessarily fair on the audience who have come to see somebody else but once their chosen act gets on the stage most of them will "forget the pain" and generally enjoy the night as a whole unless the support or floor singers have been really bad.

Many clubs may have a succession of floor singers/performers for 20 mins or even longer rather than a "warm up" act before the main guest arrives. At least, this offers some opportunity for variety even if the next singer isn't necessarily any improvement on the previous one!
:-)