When you use the Nikon MB-D11 Multi Power Battery Pack does it use power from both batteries at the same time so when you are out of power they are both dead?

How much weight does this add to the overall weight of the camera?

Can you use the grip and only have one battery either in the grip or camera installed and the camera still function or do both batteries need to be installed for it to work?

What are your thoughts, pro or con, on using one of these? I have never used a grip before.

Any information or thoughts about getting one of these or not getting one would be greatly appreciated, Jeff