Yeah, I have to assume that the reason people keep using forum spam is it generates at least some traffic or improves their google rank through seo, also it's cheap since most of them are bots.
It's probably harder to quantify the number of users it annoys and makes decide never to do business with them because I'm not sure if I've seen any spam we've ever gotten that I would have had any interest in, and I suspect that's the case for most users here. I also wonder what the affect of people posting to their Facebook wall has.
"Hay peeps I know this forum has nothing to do with this stuff but who wants some bananas from the republic to put in yo new kitchen while wearing your ugh boots that are covered in virtual gold that makes ppl say WOW"
I mean, I dunno about anyone else, but that didn't endear me to any companies even before I became a moderator.