The keys as a punchdagger trick does work, the problem is that it hurts you just about as much as it will hurt them too. It should always be used as a last resort where deescalating and running away are not going to work.

Better to go to the hospital for your wounded hand than be dead because you lacked the strength or power to fight them off with regular punches.

My point was anything is useful when your life is at stake. Not technique or weapon is perfect. Better than being dead though.