The issues with an external battery are: Phones tend to die when you're using them. This means you're either constantly plugging/unplugging, or you have a cable hanging out of your pocket all the time. My LG G3 can easily drawn than a smaller battery can supply, so it'll still go dead. A big enough battery is a large, heavy thing. It will monopolize a pocket or small purse.

From a strictly emotional standpoint, the idea that I must carry a separate battery if I plan to be away from a charger all day is just stupid.