Putting aside the issue of professional (i.e. people who you pay to listen to) performers for a moment, I would suggest that anyone who wants to perform in public (I'm not talking about singarounds with mates here) should first indulge in a little honest introspection. In my experience, most people want to give the best performance that they are capable of and don't like making arses of themselves (or getting them smacked - in public at least), and a good way to do this is to first record your performance – there are some reasonably-priced digital recorders available now that give excellent results.

Then you need to ask yourself some hard questions:
"Do I like what I hear?"
"Am I as good as I thought?"
"Am I as good as I can be?"
"Am I as good as I want to be?"
"Will people enjoy what I do?"
"Do I care?"

And, to my mind, most important of all:
"How can I improve?"

What you do with the information you glean is up to you; for me it was both reassuring and depressing, but now I won't perform a song in public without taking the test.