As I said, I only found possibly two in the past five years where it might have helped (not in the US) and even then it was predicated on the the pilot or possibly some automated system identifying that the aircraft was in sufficient peril to push the big red button. The two older crashes that Steve mentioned had definite longer time to figure out that there was a problem. Things like missiles, CFIT, approach and takeoff crashes, and arguably things like AF 447 where the pilots just went braindead aren't likely to be helped.