The one time I'm sure didn't leave a tip was when I found dirt all over my salad and the waiter acted as if he didn't care. Otherwise, I tip twenty percent. Not tipping isn't stealing because it's a add-on benefit and not pay or entitlement. Some people act as if you should tip them for merely doing what their job requires and not beyond what their job requires (like Starbucks baristas!).
You're right...I agree. The customer tip should be earned.
However, I am always surprised at the number of people who claim that 10% is the limit, even for good service.