There's such a lot of controversy about session etiquette but here's my bit! Personally, I don't mind people joining in by singing or playing (although the right key is good!) I reckon that for some people it is their only opportunity to join in, either through lack of confidence or ability, so it maybe makes them feel more part of the group. Music only sessions or singing only are fine too, as long as it is clear to everyone - particularly the new person who has walked in and could make a fool of him/herself (usually me.)
I suppose my only hates are loud talking and BO! (Oh, and someone joining in the verses with the wrong words, and people using my concertina case for a beer stand and the man who was sat next to me on Sunday that kept farting and..... ok, so I'm very intolerant really!)