Yes I agree with that aspect but it's no guarantee for good service (which was implied in his post), it's applied retrospectively.That's the thinking behind it, if someone knows they're going to get tipped then supposedly they'll work harder for it.
For a bartender for repeat custom I agree you are correct, but for a meal out it's in no way related to the experience you get as you also pointed out. It's not like you always get good service in the USA either (in which tipping is mandatory).If you tip a bartender at the beginning of a night they'll come to you first when they next see you useful on a busy night. Not sure how it would work in restaurant though.