I don't think that it's wrong to have a tune only session, but generally speaking, I don't think that we are good enough at giving clear ground rules to newcomers. It's ten times more considerate to take a newcomer aside and explaining what the story is than saying nothing and just freezing him out, which I have seen manys the time. Mc Graths description of football and cricket is very good. And I will stick to my guns and say that somebody who is thrashing the livin' shite out of his guitar/bodhran or whatever in the wrong key/tempo and murdering another players rendition has to be told (politely at first) to take it easy. Personally, I use an ultra-light pick with my guitar when I'm trying to accompany something that I'm not quite sure of. I might learn something but I won't be screwing it up for someone else whilst I'm doing it.