Take parent comment for example. I disagree with it and if it had a score of 2+ I would've downvoted it, because my gut reaction was not "extremely positive". Now however I look at it and do not know if it's 1 or 2+, and so I cannot cast my disagreement vote without potentially pushing the comment into non-positive range.
(edit) Actually... what if there was separate agree/disagree indicator for each comment. As in "I was going to say something, but this comment is exactly that -> agreed". This will turn up/downvoting back to its original role of interesting/junk quantifier.