The MB-D12 also takes the EN-EL18 battery. In fact, using that battery is the only way to tweak out 6FPS in the DX mode.
They REALLY deviated from previous MB-D grip pricing, though. I mean, almost double the price of an MB-D10 and more than double the price of an MB-D11? I am thinking that the marketing department spearheaded this as a way to lower the price of the actual D800 body. "We'll get our extra profit on the battery pack."
There is a serious opportunity for a third party to make a D800 MB-D12 alternative and make some good money.