The problem with blocking it is that reddit is not just for time wasting. It's also currently the most popular forum host for all sorts of experts, from audiophiles to network engineers. Whenever I have a technical query and Google is returning a whole load of SEO spam I append 'reddit' or a stack exchange site to the query and can fairly reliably expect to discover a technical subreddit which answers my question and where the best answers are the most visible, thanks to voting.