Nonsense spam messes with adaptive spam filters. They learn bad rules, thus it becomes harder to identify real spam.<p>----<p>Edit: I don't know how effective it really is, but this seems to be the motivation for why spammers at least try this. Many more details here:<p><a href="http://en.wikipedia.org/wiki/Bayesian_poisoning" rel="nofollow">http://en.wikipedia.org/wiki/Bayesian_poisoning</a>