All this is very strange; the battlefield appeared to be littered with dead windmills carefully erected by those who would persuade us (apparently) that neither skill nor practice is needed to sing folk-songs (or am I guilty of erecting a windmill of my own? – it's a little hard to tell who is saying what among so many people not really saying anything).
I didn't get a reply from the leveller regarding my exacting standards - that's ok; I didn't really expect one.
Perhaps we might do better with this one:

Leveller:
"Jim, I assume the actors will be getting paid for their efforts. Maybe that's where the difference lies."
Are you suggesting that the only things that are worth making an effort for are those for which you are paid?
Or how about:
"Taking Jim Carroll's argument, we should ban all amateur productions such as school plays and concerts, amateur dramatic societies, village pantos, carol singers etc. because they may not come up to his exacting standards. What a sad world that would be!"
How do you work that extraordinary piece of logic out – sorry, you can't really respond to that until you define what my 'exacting standards' are – so maybe we should give that one a bit more time.
For the record; virtually all the activities mentioned require a fair amount of developed skill and preparation – amateur or not – I know – I've attempted to tread the boards myself on several occasion, without much success .   
"It's harking back to the 'folk police' arguments we've had in the past,"
Sorry, don't quite follow this – we're not reverting to back-of-the-bike-sheds name calling are we?
"Where we appear to disagree is on the application of standards to amateur performances."
Are you suggesting that because an activity is 'amateur', no skill or effort is required.

Richard
"But how does it relate to the people who Jim and the Captain would say should not sing or play?"
Who has suggested that anybody "should not sing or play"? I haven't, nor has anybody else as far as I can see. We have merely suggested that before they appear in public they should have done the work necessary to become singers.
"A man I would not ban"
As you are the only one to mention banning anybody – who WOULD you ban?

Snail;
".......as responsible adults, they can be trusted to put in the necessary work."
Fine – and when they have put in that work and mastered the basics, then they should be recognised as singers and given places as a performers.

Sleepy Rosie
So who are the guilty parties and why do they do it?
Good question – as far as I'm concerned the club organisers should be responsible for what goes on in their clubs and be shape their policies accordingly so the music stands more of a chnace of surviving than it has been given so far.
Why do they behave the way they do?
The logic of their argument appears to be based on an extremely patronising belief that newcomers asked to put in the groundwork in order to become singers are quite likely to hoist up their skirts and head for the hills – can't think of another reason; can you?

Nick
"Jim has never defined his benchmark of acceptability"
Well; I have; but in case you missed it tenth time round, here it is again:
A singer should be able to sing – hold a tune, memorise the words, and understand the song enough to communicate that understanding to the listener.
"borderline examples"
Not really into micro-measuring singing abilities – even if I saw the necessity of doing so.
I suggest that anybody involved in singing who can't tell where a would-be singer is consistently singing out of tune or making a hames of the text really should have gone to Specsaver.
Yours in anticipation,
Jim Carroll
PS
Leveller:
Sorry; have just seen your posting - have not got time or energy to read fully and respond - (music calls in town) - but will do son when I get back.
I have to say that your description of a folk club (at a somewhat superficial glance) sounds less like the clubs I know and more like a Freemasons Lodge where the public does not venture, but as I said, a superficial glance.
We all 'want' it to be performed at the highest possible standard - surely; but that is not what is under discusssion; rather, the question here is what is the 'lowest' standard we are prepared to accept from performers at the clubs.
Our music is held to ridicule by those outside our immediate circles (in the UK, that is - not in Ireland any more) - I believe that this is to a great extent due to the fact that it is not held in enough exteem by many of those participating - if we don't take our music seriously, how can we expect anybody else to?