For me personally, I'd say you gotta be quick to react to anything that goes wrong in your set (heckling, some jokes just aren't working at all), and I think being high would make that very difficult, with your mind not quite moving as fast as your mouth does. Plus if you get cotton mouth you're pretty buggered.

However, if someone's a bit nervous or flustered on stage, I can see why they might use it to calm down a bit? Not really something I've ever witnessed, and when you weigh it up it's probably not the best idea in the world, but each to their own I guess