I think a good reason for this (since its pretty common practice) is unlike a barista, where you'll have a cup (or maybe two) tops during your entire interactio

I think a good reason for this (since its pretty common practice) is unlike a barista, where you'll have a cup (or maybe two) tops during your entire interaction with him/her, you're gonna see that bartender many times over the course of a night. Factor in the fact that the competition for his attention is much fiercer than that of a barista's, and suddenly its quite rational to do everything in your power to make sure he remembers your face.