Re: Disagree with general consensus here...
The problem with external battery packs is that they are limited in current capability, you can only stuff so many amps through a piece of copper wire and those tiny connectors before it becomes possible to light a cig on it.
An internal, removeable or otherwise, battery and holder can be designed to provide many amps of current and, if it's removeable, it can be replaced when it goes flat offering effectively instant recharge.
I used to carry about a spare battery for my phones, it's amazing how often it got used in the early hours of the morning in data centres or remote sites where it was inconvenient to recharge the phone.
Then I got a HTC One, I love the phone but miss the removeable battery and SD card as it goes flat at the most inconvenient times and I'd really make good use of a large capacity SD card in it.