When you use it, it moves posts down, which is exactly the opposite of upvoting! The nuance is lost on me. I don't see why we would be provided with something that acts exactly like a downvote would act, and then expected to use it differently than a downvote.
I also draw no distinction between "spam" and "a link to someone's blog about how they were patted down at the airport." I would just as soon read about free Viagra and watches.