When you visit a restaurant with whom do you form a contract? Not with the person who serves you.
And who is resposible for your greatest pleasure and satisfaction? Not the person who served you.
If you leave a tip then surely it should be shared with all the people involved in your experience - the cooks, the cleaners ... not just the person who placed it in front of you. Do you give shop assistants a tip when they give you good service? Why not?
I have some sympathy with that viewpoint - my brother has spent much of his life behind the scenes in kitchens. However, his take on this is that you pay for the meal, you tip for the service, and the bulk of the service is from the customer-facing element. To his mind, whilst the kitchen staff work hard, they don't have to face up to the obnoxious, self-important customers, and the tips are a sort of a counterweight to that.
I think a better system for that would be if we were all just nicer people to our wait-staff...
O.