I think if you give someone the option to downvote, there are always going to be people who do whatever they want with it. Telling someone not to downvote if they disgree with a well-presented argument is like telling someone not to post crap in the first place (or not to upvote crap, for that matter). Most will abide by the guidelines, but there's nothing you can do to stop the minority from misusing abusing it. Unfortunately, the wrong minority is enough to ruin the feature (or even the community, although I don't think that has happened here).