Headrests are NOT any help at all in an aircraft accident unless there happens to be a restraint to prevent the head moving forwards. An airbag, as suggested, would hold the head against the head restraint and so keep it reasonably still.
Impact with the earth at 200mph whether vertically or horizontally, will result in the head weighing around 200kg. The neck will snap, perhaps even detaching the head, and the headrest will do absolutely nothing to prevent it.
So stop getting hung up about MEL, DDG, etc.
They are a luxury to rest your head on during a long flight.