A mechanism to promote quality on-topic content and demote noise can be pretty valuable, especially somewhere with a high population. The original thinking on Reddit (and I've been there long enough to know) was that people would use voting as moderation, not agreement or disagreement.
An upvote was to mean "content like this belongs here" and a downvote the opposite. There were no comments at first, but it reasonably applied to them as well once they were added. Unfortunately, votes are too simple and too opaque to maintain a norm like that. Were I designing a discussion system, it would probably use labeling like Slashdot rather than simple voting.