Why is it so important that, as a tall person, my neck be as free as possible in the event of an accident? To ensure that I absolutely break my neck should we stop short or hit something?
It's important so in the case of an emergency, the tuna sandwiches Y pax usually carry on board flies straight thru and hits the bulkhead pax, because they deserve it for being seated in the bulkhead (it's in the in-flight magazine: bulkhead pax may be hit by tuna sandwiches).
If you raise your headrest that means the tuna will hit the pax seated behind you (unless it's corn fed tuna, which has different aerodynamics, but everyone knows that).