I plan to use AA batteries, which reduces the total cost somewhat.
To be exact, it's not just a place to hold a second battery. If that's all it were I wouldn't want one. The availability of the buttons and command wheel when the camera is rotated to the portrait position is what attracts me.
I may be stupid, but I'm buying the MB-D11. My experience with aftermarket products in general is that while some are OK, many are not of the quality of the OEM product, especially when it comes to fit and finish.
As with anything, the bottom like is: If you don't like the price, don't buy the product.