Sorry Will - a spoonerism - should have read regular singing session.
"I run a small session in my local pub on a monthly basis"
Don't think we have any serious disagreement; we call what you describe here, 'singing circles' - usually anything goes repertoire with basement-level standards.
Have been to a few (usually only once) and found them sometimes fun but musically unsatisfying.
My objection is to 'folk clubs' who allow, even encourage audiences to join in unasked.
My "party line" is that the folk music I know and love, and have devoted the best part of my life to, is beautiful, complex, extremely enjoyable and satisfying and important (and demanding) enough to be performed well enough for the listener and performer to go home with enough fuel to encourage him/her to come back the following week to do it all over again, and maybe even to take it further (some of our finest researchers sttarted off as singers).
Jim Carroll