"so purists do exist, and they wish to exclude any singers who wish to accompany themselves with an instrument."
Not necessarily Cap'n - in my experience the ones that did it, certainly over the last thirty years, did it to discourage Bob Dylan wannabes, simply because that was not the type of song they wanted to be known for.
I've seen the opposite reaction from anything goes clubs when somebody gets up and sings a long ballad - it wasn't what their audiences were used to - simple as that.
There were clubs in England that actively discouraged instumental accompaniments, but I have never seen one of these for at least thirty years.
I was more than happy to listen to songs accompanied by Peggy Seeger or John Faulkner, or Sandra Kerr or Jack Warshaw, just as I was happy to listen to contemporary songs made in traditional styles.
None of this has anything to do with "purism", rather it is attempting to present a certain type/style of song to an audience who has come to expect such songs.
What do you suggest be done to discourage the wannabe Dylans (or Walthamstow cowboys, as I have heard them referred to) - audition them beforehand, or should they, as some clubs did, throw open your club to anybody who wants to sing, whatever the type of song?
I'm afraid your argument appears to be based on the latter idea, which yo seem to be putting forward as a singer/instrumentalist, maybe out of self-interest.
Jim Carroll