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.
15 NS Ripsaw . 14 NS Proto HD . 14 NS Cobra . 13 Gnu Space Case EC2 . 2013 Salomon Pow Snake
14 Now Drive . 13 Now IPO
14 Flow Hylite