Tyler Cowen puzzles over tipping, noting, "1. Two studies show little relationship between quality of waiter service and size of tip."
However, what the studies show is that the service rendered (very roughly measured) does not seem to affect the size of tip left. Perhpas not, but the size of the tip you leave definitely affects your service when you return to the same establishment. Try going into a NYC bar three or four nights in a row without tipping for your drinks and see what kind of service you get by the end of the period.