Richard Bridge - sorry I didn't mean to imply that joining in the chorus, or even the last lines of songs where it has become a tradition to do so, is a problem. Quite the opposite - there can be a real pleasure in being part of that experience.

I was thinking more of those times where a singer has put a lot of thought into how they wish to express a song ( where each word and phrase is important and subtle differences in timings can be crucial) and other people hum or sing along without that appreciation of the care that has been taken.   We are lucky enough to have some very accomplished singers of traditional ballads in our area and I find, as an audience member, I can be thoroughly absorbed by their interpretations ... but not when I am hearing the tune hummed slightly out of sync from somewhere else in the room.