I am less concerned by the implication of reduced productivity. Though reading Hacker News may negatively impact present productivity, it may improve future productivity by exposing readers to new concepts, approaches, etc.

It reminds me of an article about Pixar (though a cursory search doesn't seem to find it...) which described their workspaces as having only a few central bathrooms in order to facilitate people running into each other, discussing ideas, and being exposed to different people's work. I think Hacker News provides a similar service without having to get up and walk at all.