Since its split seconds, whatever prompted the cop to pull his gun, will still be in view when his gun is pulled, i.e. the situation isn't going to change in 1/10th of a second.
That said, I talked to one of my friends who is a cop, he kind of likes the idea, but his only concern is that other cops may now pull their guns faster or be quicker to draw, just to get the recording started.
I'm on the fence on this one. In some cases it could prove the cop was right and in others it may prove him wrong. How many times have we watched a replay in sports, where the call is in dispute? Three camera's three angles and it is still too close to call.
the article says the camera starts 2 seconds after pulling the gun. In an emergency, thats time for two or three shots.