I know I'm a bit late, but...
Porn spammers actually don't care whether you look at their porn.
What they care about is whether Google indexes the site, and has a high pagerank. If so, then if they can get a porn spam link on there without getting noticed, they get the pagerank of the site they're spamming for increased a lot.
And, the bigger sites get hit just as hard - they just have more moderators to nuke the spam as quickly as possible. Also, most other forum software packages (including phpBB 3.x, actually) have automated post report functions that send e-mails to all moderators when a post is reported by a user.