It should in theory decrease police brutality and other improper public behavior by the police, plus it should also provide evidence to achieve justice much more often.
This has actually happened a couple of times with police video footage that I can recall. Essentially going, "See? This was justifiable!" If I'm the police, I'd absolutely want the camera on during every confrontation, for that specific reason.
Most of those videos you DON'T see, because either 1) the original video was justifiable on its face (deadly force was the right call), 2) it's so damning that it doesn't get released until absolutely required to do so, or 3) legalese that I can't remember right now. I think it's some state-by-state rule about releasing video footage.
I mean, there's negatives (like the white Columbia officer casually dropping n-bombs during an argument), but I wouldn't see why every cop wouldn't be for it. To me, it would reinforce their inherent "good-ness."