My experience of sessions has been mixed. Sometimes I have felt welcome, other times not. The most extreme case was where my instrument was sabotaged while I was in the Gents - not irreparably but I couldn't play any more that session.
The thing that irritates me most is the double standard of many musicians with regard to dancers. They expect inexperienced musicians to sit at the back of sessions and not cause any disruption but the suggestion that a dance should be geared to more experienced dancers (rather than the lowest common denominator) is met with howls of "elitism!".
I think the best comment was from a friend of mine while a folk festival was in the planning stages. The idea of a "dancers dance" was greated with the usual howls of "elitism!" from a number of musos. My friend, who is a skilled mashall arts student as well as a dancer, said "OK, if I can't have a dancers dance, I'll just come to some sessions and bring an accordian".
Muso: "I didn't know you could play the accordian" My friend: " I can't!"