I play regularly in sessions in a pub in Co Antrim, twice a week. The pub is not big and over the years the session grew. It gets especially big in the summer when itinerant musicians wander in. Last summer there were two session in full sing with 15 musicians in each - one in the pub and the other in the restaurant attached. However, the Tuesday session became so well established that we started a second on Thursdays. While the Tuesday session is predominantly instrumental, the Thursday has a greater mix of traditional and singing. The locals in the pub are there to listen to the music and generally when someone sings the bar can go completely silent.
We ran across a problem a couple of months ago in that the feel of the session started turning away from traditional with people joining in with "popular" and country and western songs. Now it would not have been right to sort the problem out within the pub. So we stopped going for a few weeks. If the country stuff could survive by itself, well fair enough. But it didn't, so apart from holiday breaks at the moment, Thursdays will start up again as a traditional session again.
It saddened me about the earlier posting about a singer who was booed for forgetting his lines. This happens regularly in our sessions and to the contrary, the singer is vocally encouraged to carry on and fininsh the song, and quite often receiving a big cheer for persevering.
Anyway back to the initial thread: common etiquette in Ireland is that a non musician should give up their seat for a musician so that the session sits together. If that does not work - stop playing for a few weeks or play irregularly so that the punters drop off in numbers.
I have a strong belief that sessions are for the musicians. We play for our own benefit and the best place for a session is in a pub where the locals appreciate the music. But we play for ourselves first. The music is on our terms and that without doubt makes the best session.