"One thing frequently cites as a reason is better balance but that is not really a factor. Balance of a heavy lens is not related to camera mass or weight."
Stan - Interesting comment and I do agree with you, however, I find with the grip the camera now rests on the heel of my left hand and I think it does provide better stability for the shot regardless of which lens I have mounted.
I got the grip because the camera grip is a tad small for my hand and my pinky finger kept pulling the rubber cover open. The extended grip solved the problem. I have since learned I like the grip for vertical shots, much more comfortable to hold.
I only use a battery in MB-D11, I found putting one in the camera counter productive when you have to remove the grip to change the camera battery and it seems to last forever anyway.