we have the same problem with hundreds of porn splogs linking to us.
Google do seem pretty efficient at ignoring them so their impact on
our serps is barely noticeable in Google. I think the motive behind
them is either to generate adsense revenue from scraped content and/or
a competitor or plagarist using them to  copy the contents of your
pages - which although Google will not initially index (because its
duplicate content alogs filter them out)  there will be a day when for
example your server goes down or a coding mistake is made on your
page, which enables the plagarists copied content to get indexed in
priority to yours.