Remember Contra? When you holding "Down" and hit "Jump", you're falling through "Jumpthru" platform. The solution with disabling the behavior for a second is not an option. We need platform behavior bundled with down button, with possibility to check "fall through"?
Also, "Down" button can be used for simplifying "Duck" feature, but that not so vital.
P.S. I'm not a coder, so maybe this cannot be done in a simple and 100% working way.