Sorry to have to contradict you, cap'n, but perhaps this may illustrate why inappropriate joining in can spoil things for audience members rather than professionals.

I was at one of your recent bookings. It was a great evening - in many ways just like folk clubs used to be with your own informed choice of songs containing many of my (and many other people's) favourites and also several less well known songs which were thought provoking, stimulating and very enjoyable. The chorus singing was mighty. The floor singers seemed to raise their game a level in recognition that they were part of something special. The room was packed. I had a great time. But a few songs were, for me at least, marred by just a couple of people who insisted on humming/singing along with the verses. You might not have even been aware of it. You have a strong voice and you concentrate totally on your delivery of the song, and that may mean that some of the distractions pass you by. The interference certainly did not make me think less of you - in fact I was impressed by how unaffected you were by it - but it did mean that I lost the thread of the story of one ballad because my mind was busy cursing the person a couple of seats away.