"]Real folk clubs used to expect two songs from a floor turn. Great for guitarists as you could do one with an easy accompaniment followed by one a bit more entertaining as your fingers had loosened and warmed. With one at a time, pretend to be Mr Carthy at your peril.."

This is a good point, and I think there should be more of this: two songs per round.
I can understand that you might have a room that's so busy this would be impossible without denying people the opportunity to sing. But it's always good to warm up. If you suffer from nerves, like me, then you need 1 song to get going.