I may agree with the sentiment of a snarky post and upvote it and thereby damage the discourse by encouraging snark. Sure, no karma system can prevent people

I may agree with the sentiment of a snarky post and upvote it and thereby damage the discourse by encouraging snark. Sure, no karma system can prevent people from deliberately rewarding bad behavior. But at least you won't be posting another redundant snarky post, which makes my point that using upvotes to agree helps the site. [D]ownvotes don't rob the site of a person's reasoning Sure they do. If I downvote a post I don't agree with instead of replying to it, no one else knows why I disagreed. I may have knowledge that would be of general interest to the community. Which makes my second point, that downvotes to disagree harm the site. Using karma to promote quality posting and discourage negative behavior is symmetrical; using it to agree and disagree is not.