If I have 5, 8, 11,… IPC (anything that is 3*x +2) and, if I believe an attack is iminent (ie next round). Then the AA is essencially a unit that killes half a plane and is taken as loss in the first round. So it is strictly better for defence in the next turn, and next turn only.
Possible uses: round 2 in London to stop a sealon (especially since i might have more than 30 IPC).
Round 2-4 in India to prevent india crush.
Moscow round 8, if I am not retreating, and think buying that one will make me barely survive for some wierd reason.
Egypt,since I only will have a minor IC there and might need to maximze defensive power within the limit of buying 3 units.
In total, I will consider buying them probably 1-3 times every game, and buy them once every other game.
Japan and germany might do similar calculations from time to time.