ICOT – How can you ever withhold some or all of tip to a waiter, which you said you do, based on your own comment?
I’m not ICOT, but if I may take a stab at it…
The whole concept of a tip is based on social convention. In places where it is customary to tip, one should (perhaps is required to) tip for the service provided. If you’re in an area of the world where tipping is unheard of, you do not have to tip.
A part of that social convention is that the amount of the tip is based on the quality of the service. Anyone who receives tips understands that this is the way it is (or should be). As such, since it is a part of the same convention by which tips exist in the first place, I don’t see a problem with withholding a portion of the tip when the service is bad.
The Wolf