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.
17 Korua Apollo . 17 Nitro Treehugger . 17 Lib x Lost Mayhem Short Fat . 17 Jones Storm Chaser . 16 Never Summer Swift
17 Burton Malavita Wing . 16 Burton Genesis X
16 Flow Talon