tips should be for outstanding service , something above and beyond the call of duty.
tipping anything else degrades the quality of service and as you said they get paid for that. If it wasn't for perpetual tippers the service quality in the UK would be far far higher.
Pretty much sums up my thoughts on the subject lol. My sister has a second job as a waitress in a local bar/sit down meal place and she gets tips regularly but not from everyone. She never asks as thats rude in her and my opinion but just ensures that the customer enjoys their meal and time at the establishment whilst being under her service.
I can see why some people do ask for tips though as some of the minimum wages are appauling in some of the places I've had friends work for lol