Only GoPro3 Black Edition doesn't works without it's battery. With the other models you can uses them without own battery.
Related to weight, that is a "wanna be ligther phantom", because if you take out GoPro battery for reducing weight, may be you'll get some grams lighter, and may be some longer flight time, but your main battery will power your GoPro, so that will reduce the flight time... because more juice required.... so no winner here.