I don’t think the idea is that it doesn’t matter, I think the idea is that if you disagree with someone, you should put some effort into it. On Reddit, the downvote function only really works to hide and punish opinions that the community sees as bad, which doesn’t really contribute very much in that respect. Upvotes work for indicating agreement because if you agree with a point you probably don’t have much to add to it, and if you do, you can both upvote and reply.