To be fair, you might be surprised at just how effective Mg's can be against air units.
Mgun Stats (from wiki)
Damage: 2.2
Type: Mgun
Multiplier vs Air units: 9.5
RoF: 1200 rounds per minute
Range: 700m listed, effective range is unknown, possibly infinite.
Misc Info
Damage per shot vs Air: 20.9
Damage per sec. vs Air: 418
Magazine size: 700 rounds
Continuous Firing Time: 35 seconds
Reload Cost: 50 Cbills
Note this is per Mgun, so like Zeus said, if you had a unit with any decent amount of Mguns (since they're fricking half a ton each), it would be fantastic AA. Hell, a Hollander with some mguns, BHP, GECM, and a decent amount of ammo would be fantastic AA. That amount of Mguns would be decent against BA, but it would be pretty useless against anything else.