A good session, IMO,is one that can accommodate novices and not make them feel left out. I like to think I give new people all the encouragment they need, and if someone who's been sitting watching for an hour gets out a whistle and tentatively joins in a tune, I think "Yay! At last!". My sticking point is if they want to play from written music and be told in advance which tunes are going to come up - it just doesn't work that way, and they're going to have to re-learn their approach. Written music is fine for learning at home, but it doesn't really work at sessions.

I agree with McGrath that slower doesn't necessarily mean easier. When I practise on my own I may slow down certain bits, or stop and repeat them, but doing a whole tune slowly has problems of its own. I find I need to get the pattern of the whole tune in my head, and that's more difficult of it's slowed down. Also, the techniques for playing are different. When you play slowly, you can't phrase things the same way because you run out of bellows, or breath, or bow or whatever it is that makes your notes last longer.

I don't understand how anyone expects people to become competent by just listening, or practising alone. How I've learnt most is by playing alongside others who are better than me. From what I've seen, most confident musicians who've learnt like this are happy to let others learn from them, and seeing this happen is one of the joys of this type of music-making.