It seems to me we're talking about several different things here, which partly accounts for that whooshing noise you hear as people talk past each other.

There's a difference between songs and tunes, and there's a difference between joining in at all, joining in at the wrong time and joining in badly.

On tunes, Jim seems to be saying that punters shouldn't join in at all; I think he's in the minority if so, but maybe things are different in Ireland. Most of us think people should be free to join in in sessions, but Jack's saying that some people join in (or don't join in) at the wrong time. And Will and Vic are saying that some people join in badly.

On songs, I don't think anyone is saying that people shouldn't join in at all, but a few people are saying that they shouldn't join in at the wrong time, e.g. by singing along to the verses (that's a bugbear of mine, although it does seem to be quite widely accepted). And Suibhne started it all by talking about people joining in badly.

I'm not sure that anyone is arguing in favour of people joining in ineptly or in the wrong key. (In my defence, in a big session sometimes it's hard to hear your own whistle - and a lot of those tunes are in G...) I don't think there's much disagreement about whether people should join in at the wrong time, or even what the wrong time is. Basically I think we're all agreed, mostly.

Now, about those cows. These are small...