Gerry, sessions are not only for the elite. And by definition people are generally expected to join in. But some people are so cloth-eared, or just insensitive, that their playing can seriously detract from the music. If nobody mentions it to them, firstly the music will suffer and secondly they'll never learn any better.

Novices are usually too lacking in confidence to push themselves forward in this way. In my experience, the problems usually arise with someone who has been playing for quite some time but simply hasn't grasped some of the fundamentals of the music.

I know of one player (OK, he's a guitarist, but this isn't a dig at guitarists) who only has two rhythms - one in 3/4, and the other an up-and-down strum which he applies to pretty much everything else, whether it's a reel, a hornpipe, a polka or a jig. He's playing a rhythm instrument but doesn't understand the rhythms. He also has problems recognising whether a tune is in a major or minor key, and gets totally baffled when this changes mid-tune. He is not inexperienced, simply lacking in awareness and understanding. If nobody mentions it to him, how will he learn?