The idea presupposes that third party batteries are programmed to be read by firmware of a certain version. So for instance, the present firmware v1.1 reads the BLC12 or its equivalent. But once you upgrade it to v1,5, the third party bat may not work. I know it sounds ridiculous, i felt that way too when i heard it.
I am using 2 third party bat right now with an unhacked GH2, firmware still v1.1, out of the box. It works okay, but the cam can't read battery life (old news). The Panasonic guy seems to suggest that the new update (come this Oct) will even block any kind of compatibility with third party bat, ie, yr camera won't even boot up.