One of the problem of having a blog with a Google PageRank higher than 3 is that you get flooded by a storm of spam. Be it comment spam or trackback spam it's a waste of server resources and, if not filtered, could fill your blog with tons of sex related links and more. Last year Subtext included a integration with Akismet, the de-facto only spam blocking service available for free on the net. But last autumn I received more than 1000 spam trackback in one day and more than 30.000 on the same day in my Italian blog...