[Splines]

It's mostly a matter of convenience and practicality. I can change the battery of my Nexus 5 with tools (mostly a screwdriver and a plastic prying tool), but I wouldn't want to do it away from home since it requires me to deal with small screws and needing some time to deal with small connectors that could be damaged if I was in a rush.

A common scenario is you're on vacation and taking a bunch of pictures and using GPS. You could bring your tools with you, but changing your battery involves 10+ minutes of careful disassembly/assembly and you need some space to work. You want this to be "changing AA batteries in a toy" difficulty levels.

[albertgoeswoof]

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.

[int_19h]

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.

[petecox]

My Nexus 4's fragile cover (glass back and bits of plastic on the side) doesn't seem to have been designed for repeated wear-and-tear battery replacements. A 10 minute procedure does a little more damage each time.

So yes, the procedure should be "idiot proof".