I agree that the use of any video - especially after an incident - should be carefully protected. However, I can think of a number of accidents where video footage would almost certainly have been able to unequivocably show what had happened - whereas all that investigators have to go on is (often poor quality) CVR tapes and the FDR readout.
Whilst we're on the subject of video, what happened to the DERA proposal for external cameras? I remember one of the Boscombe Down BAC1-11s was fitted with them - seemed to be rather a decent idea to me!