If they'd been good, of course, it would have worked the other way, and the free gig would have built up your numbers. "We saw this great band last night - they're on again tonight round the corner - don't miss them."

At festivals for example, if you pick up on some people you like the look of, you don't say "Seen them, that's it", you go round checking out where you can catch them again, don't you?

Assuming that most of the time you pick people who are pretty good, on balance I'd think that any attempt to stop them performing elsewhere doesn't really make much sense. And it somehow seems a bit disasteful in itself.