Just to note, there seem to have been a number of recent incidents where a flight deck camera could have shed some light on why they occurred. In particular, in the AF447 and Ethiopian/Beirut situations the pilots were believed to be pointing at certain instruments and wondering "what it was doing" or whether its value was correct. A camera could inform us as to what instrument was being pointed at. In the Polish/Smolensk crash (and some others), an issue has been whether there was someone extra in the cockpit directing things or distracting from things. Again, a camera would have been highly useful in determining if this was correct, and if so, who it was. In EgyptAir or SilkAir, a camera would have removed all doubt as to whether the loss of control was deliberate, inadvertent, or due to some system failure or external influence.
While there certainly could be privacy issues, I don't think there is much doubt that for evaluating the cause of incidents, they would be quite useful.
Ducking now ...