It's important to have the "old" data. We could keep from wasting disk space (and other people's time) if we bothered to look back and see if something has been discussed before we post a new topic about it. This will, in turn, reduce bandwidth as well because you have one person looking up an old topic rather than multiple persons posting responses to an old question... Not to mention all the smarty-pantses who will undoubtedly bring up the fact that you should use the search function to look up said old topics (which you would be unable to do if the "old" data was deleted).