And I have said many times that accepting no standards other than "wanting to" can and has led to unacceptably poor performances
Maybe it can; maybe it has, but that doesn't mean it always will. It hasn't at the Lewes Saturday Folk Club. Your analysis may be a little simplistic. There may be other factors to consider.

I go to sessions more than clubs. In the session scene, there is no doubt that standards have risen considerably over the last couple of decades (I don't except myself - I'm playing much better now than when I started). And this has taken place with a general ethos of "if you want to play, join in" - I've never heard of any session that imposed explicit standards for performing ability. So my experience chimes 100% with Brian's.

I would be curious to know if Jim Carroll has ever actually enforced the quality standards he advocates, and if so how he (or the club he belonged to) did it. Did a panel of judges rule on whether singers were up to scratch, with a team of bouncers on hand to throw out the ones that didn't measure up? How was it actually done? [Script it as if for a film, with full dialogue and body language written in.]