I'm considering purchasing an Acratech GP ballhead. It has the facility to unscrew the head and connect it to the bottom part so as it can be used as a levelling base. I must be missing something here, but as the top part has a levelling bubble I can't see the advantage of using the ballhead "upside down". I know that when used this way the panning will be above the ball, but still can't see the advantage. Hopefully all this makes sense!