I think for many (most?) people that would just result in them always voting +2/-2. Because stuff you disagree with is wrong (-1), and wrong posts don't contribute (-1), and likewise for agree/contributes. At least those are the heuristics that were preconfigured in my irrational monkeybrain.

Also when someone does say something I agree with but that does not contribute ("most grass is green") I wouldn't want to reward them with a +1 agree either, just punish with -1 doesn't contribute.