Factories now have thier OWN aa guns built in, so there is no sense in leaving an aa gun sitting around in your capital anymore, because you already have protection against Strategic Bombing Raids.
aa guns, DRAMATICALLY discourage air attacks against ground units, and 1 hit from an aa gun, pays for double it’s IPC worth.
A good axis and allies player, will use every single unit on the board to his maximum advantage. Moving your aa guns around, to provide you protection against air-attacks, allowing you the ability to stop blitz’s when desperate, and supplementing your army is critical and beneficial. Especially considering how many more aircraft there are in Global, as compared to Anniversary, or any other axis and allies version.
Just imagine for example, the implications - of having an aa gun in China, it can make an astronomical difference. No one likes risking planes for limited attacks… That, or even moving an aa gun from West US where most times it is uselss, to Hawaii to help in the defence of the victory city there, enough so to allow the Americans to hold the victory city, in the event of a determined attack.
That said, the same theory lays true, for using other types of units on the board for maximum advantage. For example, getting infantry off islands in the middle of nowhere, or off the mainland countries like the U.S. where they will never see action. And putting them into harms way where they face off against the enemy, instead of building new units of the same type back home, can really top out your countries effectiveness and total potential overall.
The whole game is a series of moving targets and oppurtunites, measured by logistical ability. A unit that is never moved, or used, for all intensive purposes might aswell not exsist. Put 30 Japanese battleships off the coast of Argentina, and tell your opponent he can never use, or move them. How much effect will they actually have on the game? None.
Rommel said it best I think, when referring to the defense of France against D-Day, frustrated that Hitler countermanded his orders. “I want the allies to know they fought an army here, not a series of stationary targets.”