One of the types I fly currently has cameras as external sensors, nice to have, but not as useful as one may expect. Suitable cargo bay cameras may be nice. I'm sure that instrument video - like HUD tapes would be of some use to the AIB/ NTSB etc, but I doubt it's worth the cost.
Flight deck crew video, what a waste of time and money. Better training and less politics is the solution, not YouTube. "If you're doing nothing wrong then you have nothing to fear", don't be so naive. As for trends in practice on the ground, I'm really not interested. I fly aeroplanes not mahogany bombers.
Check ride on every trip, don't be so juvenile. What do you think goes on in 99.99% of flight decks? For those conducting pre-meditated gross misconduct I cannot see the cameras providing any data - obviously.