Why the hell do you tip? Probably for bad reasons I'll bet.
A) You are rewarding your server for past performance - ie the meal you just ate.
B) You are trying to build a good reputation at a restaurant you frequent.
C) Its just part of going to a restaurant.
"A" is a good response. If the server made you go "Wow!" in some little or big way, or maybe just 'cause he or she is hot, leave a big tip. No prob.
"B" I really like, but then I've never been a "regular" anywhere. It makes sense to tip well at a restaurant you frequent. It's an investment in future service.
"C" is lame. Okay, most restaurants require servers to share a percentage of their tips with the cooks and this is based on their net sales, so a tip of $0 actually costs them money. Also, minimum wage (or thereabouts) alone would be unfair to the server in most cases. It's not the easiest job in the world.
BUT - for pete's sake a lot of servers make damn decent money for being on the ball on refilling your coffee cup. If you don't think there's surplus money at this position, ask how many servers resume's they have on file next time you're at a popular restaurant.
I think most of us tip - and tip pretty well - because we want to feel like "good tippers". If you're not coming back to this restaurant twenty times in the next year why bother giving that 20% tip? Just leave 10% and be done with it. If you need to scratch your generosity itch, consider who else may be more in need of your money.