Richard, in a mostly instrumental session, songs are often solos and tunes are generally for all to join in with. Also, singers tend to need/demand a greater degree of silence than musicians. I don't mind and sometimes enjoy the occasional song in that type of situation and sometimes even give a song myself.

That said there is a limit to how many times people who have come primarily to play music together want to shut up and listen. Apart from anything else, it can break the flow of a session. In my experience, a group of a dozen or more musicians rarely get really together until a few sets of tunes have been played and if you are stopping and starting for singers every other set of tunes, the evening may never get into full flow.

As I said, my own choice is to have a few songs - maybe 3 or 4 in an evening (unless it's a late night and too much beer has been consumed... when maybe there is room for more singing...) but I can understand why some instrumental sessions prefer just to have no singing.