Our local sessions are pretty open to whatever shows up. We've had singers, and enjoyed them. We had a fellow whose contribution was a story (mighty funny, too) and we enjoyed it. We've had every trad instrument at one time or another. And step dancers, and players of spoons ...
After more than two years, our session is still going strong. Maybe one reason is that we're not so wrapped up in our own little schtick that we can't enjoy something a bit different. Our lone close-minded thing: we want whatever-it-is to be an element of the Tradition (we're talking trad Irish here). We did have a guy with a double bass one time, though, and he did a magnificent job. Go figure.
Mind you, the singers, when they're there, sing _occasionally._ The story-teller gave us his story, then went back to his banjo. Our bodhranistas are sensitive enough to accent the tunes, not make it a Drum Showcase. No one--not even the Big Names we occasionally get--attempts to take over the session, they just inject their special flavor into the usual mix.
Keep looking--there's no reason to attend a session where you feel unwelcome. When you locate it, your local Decent Session will be a treasure for you.