Because if the person who modded you up had written a "me too" post instead and the three people who modded gspence down had all written "me not" posts, we would have four essentially content-free posts clobbering up the thread.

Yes, maybe you can make a point that people should either make a new point or not speak at all, because just stating an opinion may be likely to be biased. But (a) I don't think it's going to work -- saying that what happened to gspence's comment isn't what should happen doesn't change the fact that it did happen with the current model; and (b), well, I'd like to state my "me too"/"me not"! :-) Yes, if not stating opinions really does significantly debias, that would outweigh that concern, but I'm pretty skeptical about that actually happening, so the expected utility from the agree/disagree buttons wins out for me.