I agree with you,Jim.,.
even a good harmony can be distracting as a singer,because you start to think,thats good whos doing that,and your attention can wander from the interpretation of THE STORY,If it is a shanty I would probably feel differently.,if the participant singer wasnt very good.
I once had an experience when I was guesting at a FOLK CLUB,when a concertina player decided to join in with me,unfortunately our concertina chord progressions were different,itwas very annoying.
Barbara,your right ,its about respect,thesame thing can besaid about slowing down of choruses.