That’s not how I look at it, but you’re right, Karl.
In some cases, buying AAA is the most efficient for defense. In my game vs. bmnielsen, sometimes he only had 1-2 fighters that could reach Moscow, and maybe 10-14 bombers. Here, were an added AAA exposes 3 more bombers, you will get better defense with 5 IPC’s for an AAA than buying the equivalent in infantry, especially if your money is not divisible by 3.
This is not the only situation where buying AAA makes sense. And as I said, an AAA piece could draw your opponent in to take over a territory on a hit and run where he didn’t want to win, and could be huge for you as you can counter-attack.
Granted, at 5 IPC’s AAA is STILL over-priced. 4 would probably be about right.