Yes, I fully understand that, Jim. I think you may have answered my question with

The sound and function of the songs is the deciding factor for me - any evening requires a homogeneous whole if it is going to hang together and satisfy those who attend

It is the sound and function for you. Others may think differently.

I am no longer a folk club organiser but when I was we had a fairly successful club. It still is but that is beside the point. If I was still organiser there and wanted to keep my core audience, attract new members and satisfy the needs of a very important visitor, Mr Jim Carroll, what contemporary songs should I showcase and which should I avoid? No need for specifics and take it as read that you don't want anything that has been in the pop charts. What generic sound and function does the trick?