When you fight standing up, you don't get to strike on top of the head that easy. Put it in another word, if you can strike on the top of the head, it's easier to strike at more effective area.
I just don't see the usefulness of iron palm striking in the downward direction.
To be very honest, EVEN if I can strike downwards harder, it is A WHOLE LOT MORE EFFECTIVE striking with HAMMER FIST. The same motion, but instead of using the palm, use hammer fist to strike down. Believe me, I tried, NOTHING the palm can strike that a hammer fist cannot strike harder. Guess where I learn hammer fist?..............
UFC