I only disagree with the "unless" clause. If they're underpaid and the restaurant is doing it because they won't be at fault then you should tip that much anyway.
At the least they will get more money and remember you as a temporary savior for better service next time. The unless clause basically just tells me "dance for your meal, peasant"
As horrible as that is, I hope they at least don't put the milk back in the carton