@tom dial:
Although you make an interesting argument, I'd like to point out a few things.
1. Cost - The cost of body cameras is trivial compared to everything else they have. Taking the Ferguson force which has 54 officers, the cameras would run about $20,000 (US). That's far less than the price of just a single police car. BTW that city's police budget for 2014 was $5.2m, so we're talking about an amount equivalent to a rounding error. Of course, that's only one piece. Data storage runs around $60/month per camera, but, again, it's still peanuts.
2. Risk of Exposure - Why would video be any more problematic to keep safe than normal police computer files? Presumably the same people interested in those "embarrassing" situations would equally be interested in any filed police reports. Never mind that you're right in that the vast majority of video would be incredibly boring: Cop sitting at a doughnut store, Cop driving the car, Cop getting coffee at a convenience store, and on and on. Quite frankly it would be boring as hell to sift through all that recorded video to find anything juicy. Either way, just because the data is saved doesn't mean it needs to be online 100% of the time. There are many ways to store video in a disconnected state, even if it means throwing it on SSD drives and left on an evidence shelf.
3. Regarding an officer "forgetting" to turn on the device or "losing" the video. There are several potential ways of handling this. What would happen to a cop that habitually forgot their badge or gun? What if they left their belt behind? They'd be reprimanded and, if it continued, would eventually be fired over it. More importantly though you can bet that as the cameras become ubiquitous district attorneys would stop attempting to prosecute cases which didn't have video evidence. That alone would encourage a police officer to make sure the thing was on.