A basic AA unit costs 100 mpp. With it you protect 7 hexes, not all of which need have targets within. Buffing a hex costs 10 mpp and you know it has a target. So you can buff 10 hexes for the price of a basic AA unit, a net plus of at least three actual targets protected, likely more.
Buffing allows you to react on the next turn when your adversary makes known what he is targeting, something that may take a few turns moving an AA unit if you have one. Buffing you need not invest more than 10 mpp in a given turn and we all know how important an extra few mpp can sometimes be. Granted there are certain areas that are highly likely to be targeted, and an AA unit may be valuable in such areas IN ADDITION to buffs. But you cannot have AA units everywhere while you can buff every likely target.
As you can surmise, I am not a big fan of purchasing AA units. My mind on the topic is open, so if there is a flaw in my reasoning please tell me.
BTW, I like the Sedgwick quotation. July 2, 1864?
But how much damage does buffing AA do - and how much does an AA gun do? Isn't this the most important question?
Also, an AA gun has some use as combatant against non-air units - as well as a garrison.