" except in many cases the singer themselves."
If this is the case, you have to decide what you, as a club member or even just as a listener, do about it.
I believe that that below standard singing is one of the factors that helped empty the folk clubs - the decline in attendances was marked by a series of articles and letters in The Folk Review entitled, 'Crap Begets Crap' as far back as the eighties.   
There were many voices raised at the time claiming that if standards weren't maintained, clubs would suffer - whether it was this that caused it, suffer they did.
Now there seems to be open support for abandoning a lower level of expectation for anybody who sings in public.
I am totally at a loss to understand why people are prepared to encourage singers who haven't even mastered the most basic of skills to sing in public - it's been said enough hear - it isn't doing anybody any favours to encourage them to make an eejit of themselves in public, and it certainly does neither the club nor the listener any good.
Is it so wrong to expect a singer to come prepared?
Jim Carroll