Coffee and beer are both small luxuries that are relatively cheap to consume at home, but expensive to consume in a restaurant. I think MMM's take on restaurants in general makes a lot of sense: they're not to be viewed as a source of food, but as an expensive way to spend time with friends. It's okay to go to one in that context from time to time, but going to a Starbucks for coffee alone on a regular basis is a problem just as much as going to a bar alone for beer would be seen as a problem.