I think Andy is either missing or avoiding the poit
I can get
"What I have experienced, is friendly and welcoming groups of people, who perform, and enjoy, a wide variety of music "
Where does "folk song" fit into this - if it doesn't, why call yourself a "folk Club"
I can get welcoming groups of people anywhere and a wide group of material in a karaoke session
It is possible to ge an occasional gem in a poor session, but unlss you establish a standard, the likelihood of doing so is reduced considerably
It is unfair to expect those who have taken the trouble to learn the words and tune after the atmosphere has been lowered by those who haven't
One dreadful singer can ruin an entire evening - sadly, that's what the audience will remember
I helped run singers workshops all the years I have been involved in folk song and have come to the conclusion that, with very few exceptions anybody can become a proficient singer and the harder they are prepared to work at it, the better they will become

Again, I find myself in total agreement with 'Observer'
Anybody who sings in front of an audience without putting in the work shows contempt for the audience and for the songs - you owe putting in an effort to both

MacColl said it all for me in a long interview we did over six months back in the 80s

"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."
Jim Carroll