I saw "there isn't a -1 disagree modifier for a reason, -1 offtopic and -1 troll aren't valid substitutes." in a user's tag line in Slashdot today.
I think there should be such a moderation. I know I can actually alter the values of the moderation and set it to zero. Perhaps it should default to a 0 modification and users can set it to -1. I could see value in at least giving people who think other users's comments are wrong or kooks a place to vent it. This would allow those of us who are interested in the alternative perspectives a chance at seeing them more easily.