It seems to me that the guitarist that DaveO described was just plain selfish.

Dive in sessions work fine providing that people are considerate and do not simply hog the session - that is just plain bad manners.

It often happens that such folk are not so much good as fast and the two are not the same.

In a way they are the losers in the long term because they will drive others away - but then perhaps they don't care as long as their little clique can carry on - but they may also miss out on that gem of a new tune.

If I'm at a session and I think it is getting a bit frantic, I will start a slow air. That quite often calms things down a bit. I did once get torpedoed, though. I started playing Da Slockit Light and this pratt on a mandolin immediately cranked it up to something like reel tempo and completely destroyed the tune in the process.