In most of the subreddits I've seen, it's clear that upvoting for agreement and downvoting for disagreement are both rampant, and I think this is self-perpetuating. When I read a quality post that I disagree with, I would like to stick to the supposed spirit of the voting system and upvote, but I'll think twice about doing so because I know it will be misinterpreted. Giving me a way to explicitly signal one thing and not the other would break this bind.
And even if the system was still abused by 50% of people, it would be possible to offer some interesting filter settings: for example, you could favour posts with a relatively high ratio of Good Post points to Agree points, on the theory that highly agreed-with posts will probably score more Good Post points than they deserve.
BTW, I think the default downvote reason should be "disagree" because that's the majority use case for it despite what the Reddit designers intended. If a vote was accompanied by more information (offtop, funny, disagree, etc...) then I could tailor my experience better.
I think this is due to its utterly basic up/downvoting as well as its completely-unenforced TOS which state you should not downvote simply for disagreeing.
When you downvote, you should have to specify a reason (such as "offtopic") and one of the reasons should not remotely be "I disagree"