Well, my experience has been mixed. In the pub where I work sometimes people ask to come up on stage and sing a song. I usually let them, unless they're visibly drunk. But sometimes they're terrible and don't want to stop, sometimes they're drunker than I thought they were and they are staggering around on the stage and I worry about the equipment (needless to say their performance is seldom very good if they're in that state). The main problem with letting people on stage is the ham who is awful but doesn't know it and doesn't want to let go of the mike once he's got it. But sometimes it's fine. We've had really fun times, too - like the time eight visiting Russian doctors sang Russian folk songs. I never just announce than anybody is welcome to come on stage and sing, but if somebody asks, I usually invite them up to have a go.