Clansfolk wrote:

However it can be off putting for other artists (of any ability) to have people talk, or play wrong chords, or in wrong keys whilst they are performing, also many artist go to the trouble of arranging a song, intros, instrumentals etc... and may be happier NOT to have others join in (many others of course welcome other able musicians to play/sing along)

Thats a bloody good list of people who should stick to folk clubs and stay out of open bar sessions.