Great input, 'catters- and much appreciated!
I do play guitar, and have never attempted to play and sing anything which didn't welcome accompaniment of the traditional instruments, and as far as I know, I'm hardly one to try to turn a session into a drum circle, though I do love to play with real celtic musicians, as opposed to Chieftains and DeDanann CDs! (I thought that was sort of the point for the fiddleplayers, too...)

At one such gathering, I was delighted to run into Phil Morgan, who, like me, held his tongue and was a good little folksinger for most of the evening, but after a few hours, (and a few pints, to boot,) one of us, (I don't now recall which,) felt the need to sing, and most of the session-players scattered in a hurry. So phil and I swapped songs and got deep in our pints, and had a marvelous time anyhow.

I suspect that there's also a timeframe that has some effect- I've noticed that many folksingers are later owls, so to speak, and tend to show up later in the session. So maybe it's partially that the sessioners have to get home to families and beds earlier than we do...