I thought the argument against this was that then you lost volume to the casing, connector, removable cover, etc. which could be better used by a funny-shaped collection of various-sized batteries.
Do you know of any devices that don't require a reboot when you switch batteries? It shouldn't be difficult to implement. All you have to do is build a small second internal battery into the device that could keep the volatile memory alive during the switch.
But then there's no perceived need for it for 99% of users so, like many neat things, it will unfortunately never be built into mass-produced phones because it doesn't make business sense.
That same idea is apparently there in the upcoming Thinkpad T440s laptop (3 cell internal + 3 cell swappable). I really hope it catches on in other laptops and phones - I find I often go past a typical phone & laptop's battery span without a power plug around.
But if you swap batteries, then when you get to an outlet, you either need a special charger for the spare battery, or you need to swap batteries while charging.
the main reason to have spare batteries for me at least is to keep the spare batteries charged up and ready to roll. But yes this does require a specific battery charger. when i had my HTC Thunderbolt, i got a package deal with 2 batteries and a charger ( that also provided a USB port ) for < $20. I haven't looked for my S3 yet...
Effectively the same thing, and portable!