What is with the trackback spam?

Over the last week or so I have been getting hit hard with trackback spam.  All the usual suspects, sites pushing medicine, rape pictures and all sorts of other unsavory stuff.  I don't get what these people are hoping to accomplish. Typepad, which hosts my account has trackback and comment spam filters.  No trackbacks or comments are approved for the site without my approval.  I am obviously not approving any of this crap.  So what is the use?  Am I missing something?  I just keep adding IP's to banned list.  I would be interested if anyone else is having this problem.  I am determined not to turn off trackbacks, as that would give these characters to much credit.