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.
Then perhaps based on user's voting history, it could be calculated how likely a user is to always vote +2/-2, and how likely to actually differentiate between both axes. This could then be used as a multiplier for user's votes on the contribution axis.
It would probably require hiding comment scores though - otherwise it'd be easy to observe how the multiplier changes and game it.
That's not what I've seen in places it's actually used. Get a decent mix though I agree the directions generally correlate and you'll rarely have a negative in one without the other.
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.