Isn't that the whole point of discussion? Otherwise, all comments would only be contradicting their parent. The upvote count allowed us to balance disagreements (which usually need an counter-argument) with agreement, which don't need to re-iterate what has been said.
Now we get to have a lot of "+1" to counter-balance the criticism.
If we're speaking purely in terms of debate for purposes of Aumann-theoretic truth-finding, then there's many other things you can do to accede to someone else's point than by simply saying some moral equivalent to "Hear, hear!" For example, you can reinforce the parent post with additional justifications, or explore corollaries to its conclusions. You can provide an example if it's a generality, or generalize it if it's a concrete example. Someone I don't know saying "This!" about a subject I'm no expert on is just as informationally-empty to me as that same person posting a single-sentence "No, you're wrong." I see an opinion, but that provides me with no more basis on which to form my opinion.
But agreeing and disagreeing aren't the whole of discussion, either. The posts I enjoy the most on HN are neither of these—instead, they're just comments on the subject of the article, but with the ideas of their parent commenter serving as an additional jumping-off point for whatever tangent they find themselves on. When I think of something that's valuable to the discussion (and therefore deserves upvotes), I think of the parents to these posts—the ones that cause people to spin off tangents in all sorts of directions; those are what I want to see more of here. On the other hand, the expression of purely normative opinions, whether through the voting arrows or through long-winded sermons that still simply amount to "Yay!" or "Boo!", is something I want to see avoided altogether. I can form my own opinions by reading the facts in the discussion and doing research on them, thank-you-very-much—and if I don't have time to do that, then I shouldn't think I have time to form an opinion.
Now we get to have a lot of "+1" to counter-balance the criticism.