We've had classical guitar and all sorts, down Chorlton. We had someone do a spot on solo trumpet once - with sheet music.

What you think about that will depend on what question you're really asking. Was that folk music? No, not in any sense of the word. Did it make a reasonable floor spot in the middle of an evening? Yes. Is it a problem if people pitch up at a folk club and do stuff like that? No - at least, not as long as there's stuff that can be called folk going on as well.

A lack of traditional material is a problem - or at least a danger - but I don't think it's a problem that can be solved by prescriptive MCing.