For the price of an HDD, and the minimal amount of space that a forum post like this shall actually take up it isn't worth clearing it out, and as others have said it is a great historic archive and a resource for when you want to find something.
It also has other advantages, Google's page ranking system takes into account things like the amount of content on a site, and so a site with two pages is going to be ranked lower then a site with two hundred (although it does take into account many other factors).
What I normally do to keep on top of things is either mark or reply to posts that I want to follow, and ignore notifications for everything else. Then when I come on I can see what I want to see, otherwise I will look at the first (or maybe first two) pages of the "New Posts" section.