Some songs demand to be done in the parody version only - for example the Fields of F* ing Rye. The parody verse is often a way of indicating dissatisfaction with the song.

Naturally I would deduct marks for melodeons - being a guitar player myself.

I would also deduct marks for poor fiddle players joining in with unaccompanied singers and forcing them to change key (no I was not the singer in question - it was done to someone else at Tenterden). Likewise if someone is doing a song which has been arranged with a different bridge section do not trample on like a herd of elephants with a common but different version - or loudly play alternating Em D when the singer-guitarist is playing Em D Em Bm (no, those two were not a fiddle player but a melodeon player who never seemed to notice the difference and kept on doing it time and again until I gave up on the songs in question when that player was about - which may of course have been their objective).   

Unless a song has been rehearsed it can be very hard for accompanists (where that is the ethos) to keep in time and not drag, but it is vital that they do so: they must watch the singer's mouth and anticipate and that can be very hard if the singer is not used to accompaniment and so is pulling and pushing the timing. I knew an unaccompanied singer who used to request bodhran accompaniment and then spend the entire song trying to catch the bodhranist out.

The situation does differ between a concert, a singaround, a session (whether song-session aka jam or tune session) and I think a morris side's post-stand sing which is partly for the pleasure of the side (dance, beer, sing! - a bit like rugby without an oval ball) and partly for the audience and which I think would not rightly be regarded as a concert, so that it is important to sing different stuff particularly if the songs are not the sort for a lot of joining in.   

It is more important in a singaround not to keep doing the same old same old, particularly if it is a very hackneyed song. Singers who know that they do songs that others in the group also do need to be wary of consistently going for that song so as to pre-empt the other (unless it is an "annoy Little Legs" night when we all take it in turns to do songs that Wee Jock sings before he can do them, but since he knows about 400 we've never managed to leave him stranded yet)