Although one has to be careful that collecting these doesn't turn into a form of hoarding, it is useful to keep a list of useful/interesting stuff to search later. Especially for things you wouldn't have found just by using Google alone : e.g. after reading some HN posts there are a few "gems" that you haven't heard of before, those are worth bookmarking because they'll be hard to find later on.
For things that are easy to find on Google anyway, or not relevant in a couple of years time, I agree that it might be beneficial to just delete it. At some point I realized I had 5000 bookmarks, went through the titles, thrown out most and was back to a 1000 (still a lot, but I intend to prune it more).