I would speculate they make one less productive person ( for example you clearly intelligent guy, going as far as maintaining blog, imagine all that was directed to something like day trading !)
Or maybe it was intentionally ironic.
More on this next week!
But in the meantime, how many hobbies do you have with something to show for them? Now think again, is it the hobby itself or the way you interact with it? I believe WoW could be useful for mental development if a person engaged it in a useful manner. Similarly, chess can develop the mind, unless you never consider strategy or tactics and just sort of shove pawns around the board. That would be utterly useless, but it's not the fault of chess that you play stupidly.