Hmmm... I hope we can get an answer, here are my thoughts
*Logical Spock-mode "on"*
Well, thinking about the tech tree AA-wise, what is the good of having to research the lower caliber (20mm-40mm) flak techs if province AA doesn't provide any protection against ALL attacking air assets?
Taking this whole line of thought one step further... I believe you can make the point that until you research 70-80mm AA tech, you have NO chance of even scratching Strategic Bombers that fly at high altitude. What you can affect is lower flying units with the early 20-40mm calibers. So it seems a bit bass-ackwards in the game currently. Lower caliber AA has the quick-tracking ability required to shoot down low level aircraft; High (80mm and up) caliber has the "reach" or range to effectively shoot at high level aircraft but lacks the quick tracking to target low level planes. Obviously there is room for a "fudge-factor" in the medium calibers as to just when a gun tends to lose its capability to track fast/low targets.
All that aside, I would still like to get a definitive (read: programmer

) answer to whether or not AA/flak in HOI has any effect on air units OTHER than STRAT.
*Logical Spock-mode "off"*
Whew!
Pdubya