So, we're in sort of agreement then. No surprise there. It's no more than stating the bleedin' obvious. However, where we might diverge (some apparently do) is that a public display of ineptitude is fine as long as the perpetrator is doing his or her "best".

No it isn't. The place for a half-remembered snatch of a song which you're not sure what key to put it in or how the rest of the lyrics go is the shower and a bit of a tune that you have no idea where it comes from (or you've looked it up in thesession.org so it must be Irish, ha!) is your bedroom (or in my case, one end of the kitchen), till you get the song / tune, along with its attribution, right. Only then should you play / sing out.

You'd think everyone would want to avoid making an arse of themselves in public, wouldn't you? I do. And the point is, a session IS a public place, not a rehearsal room, But what I'm saying is that the entire ethos should be NOT to screw things up because that's what the music deserves. It's not just your own reputation at stake.