I'm wearing my best asbestos suit as I venture forth here. In my experience, there is often an intolerance of singing and dancing at sessions where instrumental players dominate. The downside of this, as far as sessions go, is that there is a direct correlation between the ill health of the dance and song traditions, and the robust health of the tune traditions.

As several people have pointed out, sessions as they exist today have a new "etiquette" (if that is what people wish to call it) that usually only allows for tunes to be played in sessions. There are usually separate sessions for dancers, and then again, separate sessions for singers. I know that is a Sweeping Generalization (tm), but it is pretty much the case no matter where you go on the Anglo, Scots, and Irish folk scenes. This segregation of the music is roughly the equivalent of musical apartheid, with the boundaries being patrolled by session fascists. [Won't that get them going, he says to himself.]

In my experience, it seems the instrumental players are much less tolerant of the singers and dancers than the other way around. Now, some sessions will invite a good, known singer up for a song or two. But I've sat countless times in pubs and bars where the well known singer can sit all night in a tune session without ever being invited to sing. In other words, they are intentionally frozen out by the session players (who know full well when the singer is present), who are adamnant about their "tunes only" rule of law. While people dancing at sessions is sometimes tolerated by the session players, it usuaally isn't. Just as with the singers, they freeze the dancers out by refusing to make the next set a danceable one.

Considering how healthy the instrumental session scene is, and how precariously the song and dance traditions are teetering on the brink of extinction, I would think those with a supposed "love for the music" could find it in their hearts to be a bit more accomodating to the singers and dancers. Session players, in my experience, are much more interested in their personal satisfaction level on the night, than they are in the music or the traditions. But that is just one man's opinion.