Yes. Even if I partly disagree with you, I upvoted you. Actually, I find myself upvoting comments that are in a negative territory and that present a valid point more often than good comments that are in a positive territory!

I believe we need upvotes for the reasons hugh3 mentioned. Regarding downvotes, there should be a cost, like with StackOverflow where each downvote costs you one reputation point.

Another orthogonal dimension to voting could be agree/disagree. I saw this on a national news forum and it's quite useful (e.g., 5 people agreed, 100 people disagreed).