Anecdotally, the culture gap between America and Europe when it comes to obligatory tipping is wider than the issue of tipping. Americans seem to generally value superficial pleasantness more than Europeans do. If a waitress in America is having a bad day, people still expect her to put on a pleasant face for the customers, while in Europe people seem to think it normal that a waitress having a bad day would treat other people in a natural fashion, clip and surly. For my part, I prefer the American culture and try my best to reciprocate. If I'm having a bad day, I try to remain superficially pleasant when interacting with waitresses, or anybody else. My bad day shouldn't become somebody else's bad day.
Tipping is not necessary for that sort of culture, but I do think it helps to reward it, and washes out those waitresses that can't manage it.