Reply to tootler:
"I don't know if you play an instrument or not. I suspect not or you would not have said what you did. Do you know how much effort has to go into learning to play an instrument? Yes, once you get competent, you can learn tunes reasonably quickly, but there is a lot of effort needed to get there."

I admit to some exaggeration - guilty.
But, yes, I do play an instrument and, yes, I do know how much effort that takes.
There's a difference between learning an instrument and then adding to your repertoire when you have reached a decent level of competency.
You've hit the nail on the head in your final sentence quoted above. As we're talking about sessions here then I wouldn't expect players who were less than competent to be playing in public. Surely we practise behind closed doors and perform when we are able. It follows, therefore, that players competent enough to play "out" should be pretty adept at learning tunes.
Martin