Morally, we should all treat everyone with respect.
Speaking of the wait staff in particular, if one doesn't buy any moral argument, then there's always the fact that some of the waiter's saliva may "accidentally" find itself in one's food, if one is a complete boor to him/her.
Quite frankly, given that fact, I don't know why anyone, even the most selfish, self absorbed person, would ever want to treat their waiter less than royalty. Even on my worse day, when I'm treated poorly by a horrible person, I dare not say a thing, for fear of my food getting "slimed".
Or something worse. Never a good idea to piss off somebody who will be handling your food out of your sight