This is over-idealistic. Most cases never go to trial, especially when faced with "perp's word against cop". GP post is correct - the absence of video is not sufficient alone. The cops will just turn the cameras off when breaking the law.
The cameras are very convenient, too. It makes defendants cave when they see what the jury will see.
If the convenience of the cameras is sufficient great (I bet honest officers will love it), the habit will be to always turn them on. Thus even the slightly wobbly officers will behave better, simply because it usually makes their job easier.
What do you think will happen to police officers who both have an unusual number of gaps in their electronic record, and a lot of citizen complaints? Police chiefs want fewer headaches...