If I manage to score an upgrade on a long domestic flight I'll often drink three or four beers (while watching a movie or two) as we cross the continent.
I think for me it comes down to the fact that with two little kids and a very busy life I wouldn't get any other chance to do this sort of thing.
It's not like I can sit in my armchair at home for four hours, watching movies and drinking beer, so I take advantage of it when I'm flying.
If I'm in economy, forget it. The beers are either $6 - $8 each, or, if free, you can't ever get the surly FAs to stop by with a refill.