I think the most important thing, if you're running a singers' night, is not to make the mistake of putting all the crap on at the beginning. The evening will never recover.

You can usually hide one duff performer on a guest night in the second half, last spot before the guest comes back on. (People are looking for their raffle ticket, going for a last drink/pee, etc.).

I think audiences are prepared to take the wheat with the chaff to some extent, on the basis that the chaff will gradually get better (it's the ones that don't who are the problem).