Maybe the 'organisers' should provide venues for every artform. At my local festival, it has devolved that the session (PP's definition) lovers play a lot of Irish tunes, and fair play to them, but they always take over the most comfortable venue, by persistance and volume. The rest of the enthusiasts, be they blues, trad, dancers etc., have to make do with tents or tiny corners. If it rains, they will have to put up with it or invade the 'sessionist' comfort.
Most people seem to be able to enjoy a certain amount of instrumental, but they NEED songs.
I'd says PP is right, though, about a session being musical. The language changes constantly. We used to have unaccompanied songs, (only posh people had A Capella,) and singarounds, and cracks.