That is the question.
1/6 to destroy a 12 IPCs = 2 IPCs damage on average or 1.667 it is a 10 IPCs Fg.
2/6 to destroy a 3 IPCs = 1 IPC damage on average.
Infantry can also attack at 2 when paired to Artillery. (1/6*3 IPC = .5 IPC + 1/2 *.5= Avg .75)
Still not considering heavier damage, 4 to 6 IPCs, on other ground units.
With no offensive capacity, AAA seems to have an average defense value compared to Infantry.
It is clearly not overpowered.
And, if you have less planes than AAA number, you roll up to the number of planes attacking.
The consolation is that at 3 IPCs you can sacrifice it as a cannon fodder.