I've been running b2evo for over a year now and I've only received two comment spams the whole time the site has been up. However I received a gank of referrer spam. I stop the comment spam using mod_security. Most don't bother sending a proper header so they get stopped before they even hit the site. The referrer spam only happens on weekends and only every other weekend. Seems odd really. Why do they still try even though the referrer links have been taken off a long time ago? It doesn't benefit them anymore.
Specks
General Support
b2evolution CMS Support Forums
I can't say for sure because I'm not a spammer, but check it out this way: not everyone upgrades to the latest and greatest version and not everyone applies tricks to thwart spammers and not everyone maintains their blog over time. Therefore hitting a b2evolution-powered blog in the method known to work has *some* chance of working. Given the cost to them the potential 'reward' is worth it.