The primary reason for the voting in the first place, was so that the forum could avoid all the one-line "+1" responses. The up-vote keeps that purpose. Agreement requires no explanation.
OTOH, now that people actually have to explain why they don't agree with something, the debates have brought in more people with wider views, IMO.
Yes I do see your point. Though, I tend to look the things more technical (probably because of my job). I used to consider the up/down voting as a quick view of how many people liked/disliked a specific opinion/post. A quick display of the majority's reaction to something.
Having only an up-voting button is totally blurring this view, imo.