the actual AA has a low rotation (you have to wait until the enemy plane stay in a line to shoot them), is easy to kill with rockets or just bombs, and takes time to deploy a new one if the last one is now an impact crater. even if too strong against infantry (and a lot of players steal them and waste the ammo in that), you have difficulty breaking a wing or starting a fire in the engine in the attack plane (and the fighter has enough mobility to escape until the AA have to reload), the most of the time is just luck to make critical damage in the plane.
a AA vehicle, another model of AA, or at least increase the damage against planes, the ammo, or the rotation of the actual AA.
a good option is adding temporal planes (from the deploy menu), you can choose the default fighter of the army, you can use for a limit of time (5 minutes) and then the plane leaves the battle, only having bullets (to prevent the use of that as attack plane) and do not count for the normal the fighter slot (but the team can only have one).
is just a regular option for the players that do not have space for a fighter or even if you have one fighter, can´t be enough if the enemy has 2 planes at the same time.