My opinion is likely to be rather unpopular in these parts, but in the interests of fairness, it needs to be said. I hope those who disagree with me will do me the courtesy of explaining why instead of blindly following the herd.
I disagree that police ought to carry always-on cameras. Police officers are humans too, like you and I and all of us have things we would prefer not to be recorded on video for posterity. Small but embarassing things like digging your nose, loosening your belt after a good meal, private calls with family members, complaining to friends about the boss, sharing scandalous gossip or dirty jokes... the list goes on. Everyone is entitled to their own privacy and carrying a badge should not strip you of these human foibles.
There is a very big difference between the private matters of private citizens and the actions of government employees in the conduct of their public roles. For that reason, always-on police cameras seem quite reasonable, so long as they can be switched off or set aside as soon as the officer goes off duty and resumes being a private citizen.
You appear to be arguing that government employees ought to be monitored to ensure proper conduct of their public roles. Using the same argument, we should then demand that all government employees carry a camera- after all, I want to make sure that the officer who rejected my visa application did so with just cause; ditto the DMMV lady who refused my driver's licence. Doubly so for public officials and politicians of all stripes- after all, the laws they pass or block have the potential to affect the lives of millions. My point being that, if you rely on this ground as justification, it should be applied broadly to all government employees, not just police officers.
From the public's point of view- do we really want to build a more intrusive society than the one we have at present? Bear in mind, the cameras will be recording from the police officer's viewpoint- i.e. the cameras will be recording us. If every officer is a walking CCTV, how much privacy will the public really have? Too much of public space is already being recorded as it is.
My final thought is that all these demands for cameras-on-cops is hysteria and overreaction from the public. There will always be misbehaving police officers and adding cameras on their shoulders will not change that. We should be wary that the proposed solution may be worse than disease, taking the example of how the Patriot Act was born out of a overreaction towards the 911 incident.