How do you keep the second battery charged though? Surely a power pack which charges the phone over usb is way more convienent. You can get a small one if space is an issue or a massive one if you want it to charge multiple times. And you can charge it from any wall socket at the same time as your main battery, and it can be used for your other devices.
Batteries are more convenient because you pre-charge them at home, and then you just need to swap them in the field. With an external battery pack, you need to keep it attached to your phone for long enough to recharge - that's longer than several minutes, and it kinda gets in the way of actually using the device.
