I don't. Having travelled reasonably widely, I loathe tipping systems, and find them both stressful and utterly unintuitive (particularly the "tip by default" aspect prevalent in restaurants and bars). My experience of bad, mediocre and good service is that it is entirely independent of whether the server is getting paid a decent wage by their employer.

The two biggest complaints I have with the US style system [in restaurants[ are:
* You "have" to use your designated server. Want something and they're busy ? Short of practically tackling someone else as they scoot by, you'll be ignored by the rest of the waitstaff (and I assume from the server's perspective, "stealing" someone else's customers is considered pretty poor form).
* The result of waitstaff desperate to appear "good" and thus maximise their tips is constant fawning, asking if everything is ok, can they get anything else, etc. Americans seem to lap this sort of obsequiousness up and consider it to be the ultimate measure of "good service", but I just find it annoying, if not embarrassing.

Then why tip some people and not others, despite them ostensibly delivering "service" ? Why tip bartenders but not a fast food checkout staff ? Why tip a masseuse but not a dentist ?

As I said. The only common theme I've been able to identify behind tipping is whether or not the person is getting screwed by their employer and not being paid enough to survive without tips.