If you did NOT receive good service, that should be reflected in the amount of your tip (or your lack of tip).
If you receive exceptional service, that should also be reflected.
The general rule is to tip in proportion to the service, and the quality of service being delivered.
Yes. That is the implication.
> it should be a given that the service adequately good in the normal course of business
Yes, as you say, it usually is adequately good. What about when it isn't?
Not all service is good. Not all service deserves tipping and not all service is going generate repeat customers.
As for myself, I'm all about the food. If the food is good and it gets out to me in a timely manner, I'm a happy camper, and I'll tip 20% (at a tipping-type location).
What about the times when the service is egregiously poor? You could choose to contact management. Surely they would want to know why you won't be eating there anymore... You could tip 2 pennies to communicate to the server that their service was poor... I've heard both of those suggestions.
I choose to communicate this by lowering the tip I leave, and in the absolute worst cases, leave no tip at all. Perhaps there is a better option. I don't consider leaving a 20% tip to be that better option.