Sobelena, can you please name such instances? I would have thought that the various voice/data recorders, ATC radar and voice recordings, not to mention CCTV at aerodromes (where the majority of accidents occur, or at least end up) would do the job.
If there were a tangible benefit to having video recording of the passenger cabin for accident investigators, then the cabins would have appropriately certified cameras mandated, not install some "nob-arse" in seat 7A.