Someone please correct me if I'm wrong...but if my memory serves me correctly, when I was a kid I was told/taught to keep your mouth shut and simply shake keys on key downs when we are on offense....not when we are on DEFENSE. When we are on D we make as much noise as possible...and that doesn't include KEYS!
KEYS=OFFENSE
YELLING and SCREAMING=DEFENSE
While I agree with you in principle, the keys are important on defense because if you pull out your keys and start shaking them and yelling, you can induce the rest of the crowd to start making noise earlier. I wish we could maintain the crowd noise all the way through an entire drive (i.e. not stop making noise until the offense breaks the huddle for the next play), but the keys are helpful to start the noise a little earlier on third downs.