"Whats the problem with learners who bring there music up on stage with them and stumble through a song?"
If you don't know why it's unwise to encourage incompetent singing, I very much doubt if there's any point trying to explain it.
The problem is that your audience has to listen to it - but that will soon sort itself out as, if they have any sense they'll piss off and find another club where the singers can sing.
Jim Carroll