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.
18 Snoplanks Asym Fish . 18 US Orbit . 18 Arbor Terrapin . 17 Lib x Lost Mayhem Short Fat . 17 NS Maverix . 17 NS Twenty Five . 16 NS Swift - 14 TJ Brand Retro Fish
17 Rome Katana . 17 Burton Malavita Wing . 16 Burton Genesis X
16 Flow Talon