PS DF Donars are pretty vulnerable to 'Mechs with decent heat dials and Reflective or Hardened armour. Or Geoff bekker (come on, hit me for 3 - I DARE YA - then try and get outta my arc, Hahaha).

The AA vehicles are then useful for attacking already damaged VTOLs and making your opponent sink orders into them to get them to safety when they are otherwise not viable. AA units have such reach that if they were TOO good then they would actually be unbalancing because they could quickly eliminate damaged VTOLs and earn you VC1 and deny your opponent VC2.

These rules for firing into BC mean there is nowhere for damaged units to hide - not even alongside an enemy salvaged piece!!!

That is why an AA 'Mech would be a baaaaad idea - especially wth the Assault order!