IF the emergency contact details are correct - many parents / guardians use a standard form that may not have been updated for a while. Equally, as discussed before, some parents / guardians can be "forgetful" about some medical conditions if they think it will preclude little Johnny / Josephine from taking part across the board.
If cadets (all or just a few) are away from a central camp location, with one becoming sick or injured, then do all the forms get taken away for the event's activities? I doubt it. In the worst case, major accident travelling to/from an event / camp, would the forms (if co-located) be available / readable? I doubt it. A 3822 with each cadet is by far & away the most practical solution. Many cadets (& adults!) can't even remember relevant contact 'phone numbers anyway - all stored on their own 'phone. I suggest to all cadets that their emergency contact information should be always on their 'phone - just in case of any event, at cadets or otherwise.