Nofollow Tags in Spam

I like to keep an eye on the link spam that comes in through blog comments and emails to keep tabs on what the spammiest people think is the next loop hole in the search engine algorithms.

Lately, I've been seeing a few coming through with "nofollow" tags on the links. I'm baffled as to why someone would go through the effort to spam me, and then block any possibility of getting credit for the backlinks.

My theories:

• They want direct clicks. I'm highly doubting this one, since I don't know why anyone would click "hot asian xxx" on a craft blog. Maybe it's a cross-stitch pattern that's all the rage in Japan??
• They are targeting the less sophisticated search engines. Maybe they're testing the waters in ranking on the second tier search engines, and trying to pick up the visitors that don't flock to Google and Yahoo. Although, why not just leave the "nofollows" off on the chance the big guys don't see right through it?
• They think "nofollow" isn't the kiss of death for a backlink. With government sites, many business, blogs, wikis, and new social networking sites using "nofollow" on all links as a rule to discourage spamming, are the search engines going to have to start taking them into consideration? If search engines want to give credit to the sites that get the most attention from people, they're going to have to find a programmatic way to deduce what is spam and what is legit. Good luck with that.

Any other reasons why my good friend would leave me list of links to interesting places with "nofollow" tags?