With me, it depends heavily on the situation. If it's a jam and the announced focus is bluegrass, then I am attending with that expectation. If there are serious musicians present, I'll expect a general unspoken agreement as to the boundary conditions. If it turns out to be a more eclectic group with interests and instrumentation beyond bluegrass, I'll usually stay and have a fine time playing what ever comes up. Of course not everyone feels that way. One time at a small public jam, I absent mindedly ran through some blues licks while waiting for someone to kick off a tune. One particpant let me know right then that none of that music would tolerated.
If my employer plays the sax and wants to sit in for one or two tunes, I can handle that as long as it's presented as a lark. We would always see to it that such an epmloyer got a big hand and felt proud. You can't let that go on very long because it drives off the people who thought they were coming to hear bluegrass.
Bluegrass festival jams are the hardest to control. If a harmonica player walks up there's not much you can do except move to another key and hope for the best. Usually the folks who began the jam suddenly remember there's someone on stage they want to see and walk off.
Around here you might have to drive fifty or a hundred miles one way to reach a private jam session so everyone is pretty good about telling all the guests in advance if there will be non-bluegrass elements represented. Rarely does anyone bow out for that reason but at least the expectations have been set.
It isn't that other musical forms aren't as beautifully exciting as bluegrass, of course they are. But one doesn't bring a bowling ball to a basketball game.
Banjo Johnny spoke of competition. That's one thing I really don't understand. Why does there need to be a champion band of any variety and what does that even mean? Which professional bluegrass band is best? Which of the worlds great symphony orchestras is best? It depends on who's in the outfit "this week," what they're playing and mostly it depends on the tastes and sensibilities of the listener.
I know of a bluegrass music association that holds contests and made up rules governing what instruments could be allowed depending on the number of band members. According to their rules, Ralph Stanley's band would not have qualified, same for the Sullivan Family. Go figure.