Good point Will, but it leaves the whole thing a bit open. You and your mates have set up, say, a klezmer session in a pub. You've been playing together for a while, you know the way you handle the stuff together, you exchange ideas on new stuff or different ways of doing the old stuff. Then a new musician comes along, gets out a keyboard, and starts playing arrangements of Beatles songs. He isn't interested in anything else. You're in a public bar; he's as entitled to his taste in music as you are. You'll probably tolerate him for a while (this bloke, for argument, is VERY good, and very pushy), but you came here to play klezmer primarily. Do you simply accept it and abandon the session, or attempt to dissuade him? And what's the difference between that and an "invasion" of, say, saxophonists in an Irish session? I don't know where you are, but in my experience most sessions aren't advertised; they happen by a more-or-less informal arrangement between a musician or two and an accommodating landlord.

As far as I've seen most sessions will accommodate beginners or musical strangers as long as they are reasonably well behaved, but there sometimes does come a point where the original point of the music starts to get lost. At which point, the decent musicians drift away (usually without leaving a forwarding address), the session goes on for a while, but gradually gets thinner and thinner, and eventually dies altogether. The landlord gets a big screen TV and puts football on, and another venue is lost.