My extremely brief foray into sociology in the 80s taught me that describing the batantly obvious can often bring one's attention to points that are so obvious as to be unconsidered ... so here goes:

* When a singer sings a song, no-one can join in except in chorus lines (if any), because every verse is different, and only the repeated lines are join-inable; the song is of fixed length and stops when it reaches the end.
* A tune, having no words, is the same all the way through: everything is repeated, and everyone can join in all the time; it goes on for as long or short a time as the players like (American style), or convention indicates (GB style).
* If a player wants to play in a song session, s/he will be essentially a solo performer; if a singer wants to sing at a music session, s/he will be preventing everyone else from joining in.
* Not all singers can play an instrument, and not all musicians can sing.

On the whole, I feel it would be rather unfair for a singer to expect singaround treatment at a music-only session: that's not what the musicians are there for. As a singer, I have to agree that it really is a self-indulgent ego trip!

Back to the blatantly obvious: if we all impress the distinction on our friends (and enemies!) there will be fewer occasions for misunderstanding and falling-out.

Steve