Well, I don't feel obligated to tip. Examples, in the town I go to school, there are a lot of little restaurants that don't have you order before you sit and then somebody comes a gives you your food after you sit, and that's it. In those cases I don't tip. I also don't tip in every service situation. When I do tip, usually at restaurants, I tip according to my level of service.
Having said that, anytime I go to Europe or South America, I appreciate the effect that tipping can have on the quality of service at restaurants. And, it's not even like a 'spoiled american' who confuses service with subservience. I'm talking about just outright negligent, incompetent, unprofessional, and rude service seems to be the norm. Well, at least in the countries I have visited.