I have to agreeI don't see where having GPS enabled uses much of anything. If you open an app, like Maps or Navigation and the GPS becomes active, then sure, its going to use power. But I would bet that it would be minor compared to the display being on all the time and 3G or 4G downloading maps while the vehicle is moving. Thats why it needs to be plugged in.
I leave mine enabled and see no battery impact at all.