My reasoning is the same as already stated: WoW is cheap entertainment, even if you play just one evening per week.

You can run your WoWs on super flashy PCs and have some awesome screen arrays or go the cheap route and grab some obsolete hardware to run your other sessions as I did.

The bigger question is about the time spent. I like to think that our hobby is just like nearly anything else clearly beating getting drunk in front of the telly, which generally seems to be an accepted way to spend the evening.