Yes they do have to live too but if they're providing lousy service like Bruce said he got they don't get much.
Now if I see one poor little gal (seen this many times) running like a chicken with it's head cut off trying to take care of 8 tables at one time she gets a real good tip even if she is a little slow to me. I also tell the manager how they need more help.
I tip according to the service I get. And if there is a problem I talk to the manager if the waiter/waitress is no help.
My tips range anywhere from 10% (lousy service) to 30% (super service).