Nicely turned insult Jim; the wit is properly appreciated! I said at the beginning of my post that I actually like unaccompanied singing, and to expand; I especially like 'narrative' ballads, however long they are. The poor quality of the singing gets in the way sometimes, but I agree that the singer should choose the length of the song. However; in a crowded session with limited time and lots of people who want to sing or play, it would seem to be good manners not to choose the longest and slowest song or tune you know (or in many cases, almost know).
I sing and play in a lot of sessions, but the best ones seem to have someone who, however informally, is giving structure to the evening. Someone who is keeping an eye on length of song, variety of delivery and opportunity for singers or players to participate. These sessions often have gentle rules, like 'join in if you are asked to, but not unless' or 'join in unless you are asked not to'. The very best session I play in is like this, but with lots of banter about performances as well; as soon as your performance starts to be 'slagged', you know you are doing ok!
To the button; I absolutely agree; this is a discussion about good manners and civilised behaviour, but it's also a discussion about musicianship; having sensitivity to the needs of other singers/players and to the needs of the listeners, if there are any.
Tim