So they will have to be on all the time. Make up your mind.
No the cameras would have to be on anytime they are interacting with the public. We don't need six hours of footage of someone sleeping in their patrol car.
For police haters it won't make any difference. If the cameras don't show what they want, they will just make up excuses why they don't.
We've seen what cops do when they know body cams are on or they are being filmed, one can only imagine how they behave when they aren't being recorded.