Thing is that cameras work both ways. How many times has someone, here, said that police should be forced to use cameras during stops so that their actions can be justified? They not only provide solid evidence for prosecution; they provide protection from abuse of authority. If a camera is present, but not used, it also raises the question as to why that was the case.
Taser has created just such a product. I believe some police forces in the states have started using it, and I can see this catching on and making it's way north in the future.
http://www.theverge.com/2013/4/5/4162478/tasers-axon-flex-cop-camera-takes-aim-at-privacy