I've also noticed that comment spam seems to actually work, and have seen mention that nofollow links do contribute to pagerank, although I'd guess they'd contribute less than follow links.
My pet theory is that with any filter there is always some stuff that gets through and if you're automatically spamming a few thousand comments then even a tiny percentage is going to make it worth the effort.
There is also a chance they'll get a few comments on sites with dofollow links, of which there are still some around.
There's usually a intial period where sites with zillions of spammy backlinks will rank, then they will crash after a weeks /months and will probably never resurface again.