Displacement On Demand or DoD/AFM is when the engine shuts down 4/8 cylinders under lighter loads. It's one way I get to drive a V8 and still get nearly the fuel economy of a 4 banger. When I hit the gas a little, it's like I get an extra gear and a crapload of power when the other 4 cyls turn on.
So it's not just marketing, its actually a feature that saves money. I actually want to tap into the GMLAN and see if I can force activate it deactivate it at will. Driving below 15mph will always have you in 8 cyl mode so the exhaust sounds good when you drive by the "homies" at low speeds. I'd rather idle with 4cyl if I could.
As far as the battery goes... I'd rather not modify anything in the engine bay because I'm really not the kind of person that could do it without screwing something up and ending up in the shop. Plus, I need additional room to mount the relay and the fuses. And if I decide to throw in a turbo, etc later on, i still have a little room to do it.