@Too late for AF447
It's not worth it because crashes are so very rare. There is about 1crash/decade for airlines you would fly with.
Most crashes are pilot error on take/off landing, accidents in the middle of nowhere that aren't bombs or over enthusiastic superpower air defence are very very rare.
The most useful data for an accident investigation is normally cockpit voice.
The reason the CVR only records the last 30mins is the same reason that pilots are supposed to be serious below 10,000 ft on take off/landing. It would make flying much more dangerous if the flight deck was continually recorded.
Imagine flying a plane for 8hours where the most important thing in your mind is that anything you say will be taken down and can be used against you by your bosses, your country's police, the HR department or the lawyers for a hostie wanting a $million sexual harassment suit payoff.