I personally don't have a problem with people who don't tip because of lousy service. I worked in a bar for a few years and got overwhelmed or flustered and didn't get some guy his beer EXACTLY RIGHT WHEN HE WANTED IT. Big deal, I made up for it with regular customers and the ability to throw people out if I didn't like them. Sometimes wait staff gets overwhelmed. But this is usually offset by the customers they can really concentrate on when they're not overwhelmed. (Underwhelmed?)