It would be really useful, if we could access vehicle events log during flight/after crash.
Currently it's often hard to deduce what have caused some of the fireworks.
By events log, I mean something like:
[0000.00.00 00:00:00.0000] Flight data recorder initialized.
[0000.00.00 00:00:10.0200] Part X was destroyed by mechanical stress.
[0000.00.00 00:02:12.0302] Part Y was destroyed by aerodynamic forces.
[0000.00.00 00:05:02.5001] Part Z was destroyed by collision with the ground (coordinates: 20.5452, 35.543543).
[0000.00.00 00:15:15.9082] Part V was destroyed by collision with part B.
[0000.00.00 00:17:14.0152] Decoupler 13 had been activated.
[0000.00.00 02:56:04.6852] The craft has left Droo atmosphere.
[0000.00.00 03:12:54.8952] The craft has reached a stable orbit around Droo.
[0000.00.00 14:52:16.8752] The craft has left Droo sphere of influence.
(...)
[0100.00.00 14:52:16.8752] The craft has reached DistantStarMock12345 sphere of influence.
Crafts launched before implementation of this log will simply start with "Flight data recorder initialized." having a non-zero timestamp.
Bonus: It may be also used to track back game events retrospectively to find out what was the first mission to reach some planet, etc. After long gameplay it's sometimes hard to recall such things. I had a KSP save with over 100 flights active and you can probably guess how hard it was to figure out things sometimes, even though I have noted all major missions' achievements on flag plates when I planted flags.
Bonus 2: Maybe it would also be useful for the development team, as it can be used for testing/bugtracking purposes.