Agree totally with Mr Happy although it's good to have a regular nucleus to keep things going.
For me music sessions have been ruined by the tendency to go around the room like a singaround (tunearound?). Of course it's fair that everyone gets a turn but the opportunity to join in, learn and improve by playing along with other, maybe more experienced musicians, far outweighs the benefit of playing the tune you've been working on all week and isn't quite ready for public performance in my opinion. Of course you can do this as well and a good session will encourage this. And - shock, horror - there was singing AS WELL!!
The format has become stilted and toe-curlingly self-conscious stifling any spontaneity. Sessions used to be gloriously haphazard social occasions with lots of banter and now seem more like meetings for the terminally politically correct.
I used to love coming away from a session sweaty and laughing but now feel lucky if I've played three tunes all evening - depending on the size of the gloomy inevitability of the circle.

Oh dear that's a rant isn't it? Maybe I'd better put a hard hat on now and take the flak.

Martin