I think that Steve's problem is that he is stating his personal opinion as if it were fact, and he's insulting lots of good musicians while he's doing it.

Here's my opinions: classing all percussionists together is a sign of someone without much sense, musicality, normal politeness, or experience. Holding a session in a public place and then telling musicians they can't play along is rude. Go home and play with your mates if you want to be exclusive. Or start a band and get a gig. Except it sounds like that's what you're doing. Why do you call it a session?

Here's some more: any tune that has more than one fiddle playing has one fiddle too many unless all the players are playing every pitch perfectly. Two whistles being played in unison almost always produce a loud disharmonic overtone. EVERYONE except the guitar player should shut up during the verses of a song. No one should ever play in public until they are quite competent. Not at sessions, not busking, not at sing-arounds and open mics. Stay home and practice until you are playing something that anyone might want to hear.

I agree that non-musicians who want to join in often take up percussion. This does not mean that all percussionists are non-musicians, or that non-musicians can't learn to be musicians. I once dealt with an impromptu session that had four non-musician percussionists and three other instruments. I put down my cittern and gave them a percussion lesson, mostly having to do with playing quietly enough to hear everything that everyone else was doing and respond to it. I think they learned some new concepts. The session was much more enjoyable after that. And I didn't have to tell anyone who was trying to play music to get lost.