Well the battery bacpac is supposed to give you up to 2x the power. So basically it's the same as having a spare battery. I think it would be better to keep as many spare batteries as necessary in your pocket/pack so that you don't have to add weight/bulk to the camera. It only takes a few min. to change out right? Unless you're doing something that you need to shoot uninterrupted for a long time, shouldn't be that big of a deal.
14 NeverSummer Proto HD . 13 Never Summer Cobra . 12 Gnu Rider's Choice C2PTX
13 Now IPO . 12 K2 Formula . 11 K2 Formula
13 NB x 686 580