For a commander, having an anti-aircraft gun on the position and not having one are two different things. As for whether this anti-aircraft gun is useful or how good it is, those are considerations for after acquiring it.
The allied forces indeed lack a large number of anti-aircraft weapons, especially those capable of threatening Tang Army's fixed-wing aircraft.
If played cleverly enough, quad-mounted or dual-mounted anti-aircraft guns, when concealed, can actually pose a problem for Tang Army's armed helicopters. This is evident from the battle damage figures: the most lost by Tang Army are Hind armed helicopters, followed by Thunderbolt attack aircraft.
Both these aircraft emphasize close support, especially the armed helicopters, which often penetrate enemy positions for direct output, thus having a significantly higher chance of being shot down.
