Istn't swapping a less frequent use case (other than with a camera) and covered by power banks (e.g. in a back cover)?
Very funny. The people who need to swap understandably use more power and cycle their battery more tthan a typical user. Charging one battery with another takes time, is extremely inefficient, and does not distribute battery wear. Because the phone thinks it is plugged in it draws more power with a brighter screen, longer time before dimming, WiFi on even when locked, etc. It's almost as inconvenient as plugging in and is very awkward, especially when you wear it enough to spend 50% of your time tethered to it.
Charging a battery externally reduces wear and tear on that all-important charging port that often gets damaged while plugged in. Dealing with an awkwardly dangling external battery pack doesn't help with that. Just ask yourself while you are fighting to hold those two together: why can't I just swap this dangling external battery into my phone? You get more efficiency, more life, and less cycles per cell, faster charge, and so many other benefits. It's MADDENING that there is no good reason.
When I played games on my R800at Xperia PLAY 4G ("Playstation phone" with built in gamepad) it would run dead in a few hours tops and then I was left out of touch for the rest of the day, so I didn't play games on it until I got a spare BST41 battery and EP900 external charger. My graveyard shift coworkers STILL ran it dead so I got a third battery. I didn't think it was safe to make the sleepy motorcycle ride home in the morning without a working phone, so I couldn't use my phone for that purpose without a spare battery.