A lot of people seem to be worried that they might get to experience an under-rehearsed performance.

How about the problem of encountering a robotic performance of something the singer got note-perfect 30 years ago and hasn't changed any detail of since?

I used to go to a club where the most consistently entertaining singer always performed from a book. His specialty was very funny and very verbally complicated Victorian music hall. He hardly ever did the same one twice, and every time he got the laughs. I doubt if even Marie Lloyd could have held all that in her head at once. At the same club, the most bumb-numbingly dull performer was somebody who had memorized four or five singer-songwriter numbers from the 1970s, did them in exactly the same way every time, and never did anything else.

I'd rather listen to a fumbler than a technical bore any day.

Maybe that club needs an additional note saying you don't do the same number twice in the same year, unless you get multiple requests for it.