I have 2 types of projects: one type is based on using UAV, and for this one, I agree, the battery life of the camera is clearly not the bottleneck (I have about 10 minutes of safe flight. acquiring photos @ 1 sec interval corresponds to about 600 picture).
The second type of mission is done with manned fixed wing aircraft or helicopter. On these mission, I fly a few hours (up to 7 hours). I'm not acquiring photos during the whole flight, but the camera is mounted outside of the aircraft, and I don't have direct access to it, so it has to be in standby mode at the begin of the mission, and allow for remotely controlled capture when we arrive on the area of interest.
A mission can involve a few thousand pictures (on the Lumix GM1, I can store 7100 JPG images on the 64 Gb SD card)
That's the reson why a definetely need an external power supply.
Another question to experts:
Do you know a camera allowing for fast acquisition (say 1-2 second interval) of long sequences (say 100 images) in RAW mode rather than JPG ? On my EOS 600D, I can acquire about 10 images in fast sequence, then the buffer is full and the acquisition becomes sluggish.