There is no reason why a club organiser cannot say nice or encouraging things to a would-be performer while at the same time letting them know that they need to improve to deserve a floor spot on a guest night at the club.

I'm not saying that the floor spots on a guest night need to be of professional standard, and I agree that anyone going to a folk club needs to understand how the format works, and that it may be a bit different from anything else they've experienced. But it's not unreasonable to expect that the floorsingers should reach a certain level of competence. Singers nights are a different matter.