The barber one always gets me.

That's a direct 1-1 transaction. You are getting a hair cut, so if it's good, you'll come back. If it's bad you won't. Charge what it costs and give the best service possible.

Granted if they go above and beyond maybe I would tip (buy yourself a beer as you say, but it would have to be more than "they cut my hair").

That's the slippery slope though, whereby if you don't tip you are deemed to not be happy with the service, and so the emotional imperative is to tip to not offend.