I've avoided playing World of Warcraft because many people enjoy it so much that they neglect other things in their life.
Does that make sense?
How about cocaine?
How about sex? I hear that's pretty good too.
ADDED: Lots of interesting discussion, but no one is getting at some points of particular interest to me. Most answers assume that you have important stuff to do, and you need to decide whether WoW will prevent you from getting that important stuff done. They also assume that your brain usually errs on the side of telling you to do "non-important" stuff (WoW) at the expense of "important stuff".
One question is whether there is any evidence that your brain is biased in this way. I think your reflective self greatly overestimates the probability of success at the "important stuff". I have worked very hard, twelve hours a day, 7 days a week, on "important stuff" for most of the past 30 years. The important stuff never pans out. So it appears that when my brain told me to play Freecell rather than work on that important paper on artificial intelligence that got pulled from the book the day before publication due to petty office politics, or to watch Buffy rather than do another test run of the demo I spent three months preparing for DARPA that no one from DARPA ever watched because the program officer was too busy to supervise his program, or to go hiking instead of spending another weekend working on the project for NASA that was eventually so big and successful that my boss took it over and then tried to get me fired1, or to go dancing rather than work on the natural-language processing approach that got shelved because my boss felt it emphasized the skills of mathematicians more than his own, or to LARP rather than put in another weekend on my approach using principal component analysis for early cancer detection that it turned out some guy from the FDA had already published 6 months earlier, or the technique for choosing siRNA sequences that a professor from George Mason already had a paper in press on - all those times, my brain was using a better estimate of success than my reflective self was.
Another question is why the "important stuff" is important. Fun is fun. On the surface, we are saying something like, "I have a part of my utility function that values contributions to the world, because I evolved to be altruistic." If we really believe that, then for any contribution to the world, there exists some quantity of fun that would outweigh it. And people use language like, "WoW may be fun, but it has little lasting effect." But when you contribute something to the world, if the relevant motivating factor to us is how our utility function evaluates that contribution, then that also has little lasting effect. If you do something great for the world, it may have a lasting effect on the world; but the time you spend feeling good about it is not as great - probably less time, and a less intense emotion, than if you had spent all the time accomplishing it playing WoW instead. So this question is about whether we really believe the stories we tell ourselves about our utility functions.
1. He got to award himself all of the department's yearly bonus money that wasn't awarded to his subordinates, so any obvious success by his subordinates was money out of his pocket.