Also, with all 8 bases up, you can have up to 24 installations. Since you probably wouldn't need any radar sites for total world coverage, you might have as many as 20 AA sites. If they could equip anything more powerful than SAMs, the aliens wouldn't be able to fly anywhere any more. Maybe if advanced air interception gets in, air combat will need rebalancing, and allowing improved AA sites would be a good way to do that. Time will tell, I guess.