I suppose one advantage of a bad singer is that when you're the following act, you've got to be an improvement on what went before! Seriously though, I think most clubs could manage with one poor performer but you really wouldn't want more than one otherwise they tend to drive away some of the audience, especially the non-regulars who may have just dropped in to see what the folk club is about. When the folk club is trying to attract a larger audience, it helps to put on a decent evening's entertainment and a poor performer is not going to help the cause. I know that most of us started out as nervous newcomers, but it really is a case of putting some effort into practising and improving with time. A poor performer who makes no effort to improve is basically just being rude by inflicting themselves on a usually courteous audience.