"As long as there is no pretence of only having professional level performers , no one is being cheated or having their time wasted."
Depends on how poorly the performers perform.
I've been to far too many clubs where the standers is far below anything that would be accepted in any other musical genre.
I have come to believe that the acceptances of poor performances stems from a basic contempt for folk music.
I'm always reminded of something MacColl said in an interview, when these arguments come up
"Now you might say that working and training to develop your voice to sing Nine Maidens A-milking Did Go or Lord Randall is calculated to destroy your original joy in singing, at least that's the argument that's put to me from time to time, or has been put to me from time to time by singers who should know better.
The better you can do a thing the more you enjoy it. Anybody who's ever tried to sing and got up in front of an audience and made a bloody mess of it knows that you're not enjoying it when you're making a balls of it, but you are enjoying it when it's working, when all the things you want to happen are happening. And that can happen without training, sure it can, but it's hit or miss. If you're training it can happen more, that's the difference. It can't happen every time, not with anybody, although your training can stand you in good stead, it's something to fall back on, a technique, you know. It's something that will at least make sure that you're not absolutely diabolical         
The objective, really for the singer is to create a situation where when he starts to sing he's no longer worried about technique; he's done all that, and he can give the whole of his or her attention to the song itself, she can give her or he can give his whole attention to the sheer act of enjoying the song".
Clubs should no more be places where singers bring insufficiently prepared work to practice on the audience - this is not a demand for "professional standards; just plea for respect for those who have taken the trouble to turn up.
I don't think crib sheets are any more acceptable in folk clubs as they would be anywhere else - and that has been made doubly true by the use of mobile phones and tablets.
Jim Carroll