Germany is really the only country that sould buy aa for rockets: The strategy being that on the first turn they get rockets and buy 2 aa guns, and move the german aa gun to east euro and the south euro aa to the balkans. The next turn they will have 5 aa guns capable of hitting britain, caucasious, and russia, the balkans and east euro one shoots at russia, the south euro shoots at cauc, and the german and west euro ones shoot at britain. Each turn the allies will lose about 17.5 IPC on average, AND on the first turn you can take away 7.5 IPC from gb and 3.5 IPC from cauc averaged.
11+17.5=28.5 IPC…and on your first turn if you buy 3 tech for rockets and 2 aa guns by the second turn your little investment has already paid for itself, and all of this is not cheating because only one aa per territory is shooting, which follows the rules.
I did this last time I was germany, and I won (except I spent 4 tech dice, so actually it wasn’t until turn 3 that it more than paid for itself.)
The only prob I see with this strategy is that if you dont get rockets it could be by turn 4 or 5 that the rockets pay for themselves, and even when I did get it all on first turn I spent so little on actual combat units that I had a somewhat tough time versus russia.