From the horses mouth about why have a fixed battery
"With these two goals in mind, we ultimately decided on a 3100 mAh non-removable battery for the OnePlus One.
The decision was clear for us. A removable battery would have meant adding a protective layer to the motherboard as well as extra circuitry, resulting in a smaller battery (2500mAh, 20% less battery juice!) or a significantly thicker phone.
With our configuration, the battery will last long enough to get even the most active users through an entire day of use without adding bulk to the overall build and design."