Depends. If they're getting in the way in a pool hall, well, they're just inconsiderate or oblivious. If you're playing a table in a crowded bar room on a Friday Night...honestly, then you should probably reconsider playing a serious game and keep a cool head.
Friday, Saturday nights...the bystanders arent focusing on your shot, they're trying to mack with the drunk chick on the other side of the bar, and if need be, half of them will walk over top of the table to get there. On top of that, they'll probably stuff a few of the balls down their trousers in a pathetic attempt to impress. I say, be prepared for anything.