I'm not sure if anyone has actually tried this, but there's a MK-2 variant with 4 UAC-5's and 2 ATM-12's.
It is probably the best vehicle for AA in the whole game. Just the ATMs are enough to cripple anything that flies.
You're referring to "Seige Engine"...they're ATM9's, and they are decent at AA but again, I wouldn't want to be in it if a reasonably good Shiva pilot was in the air and gunning for me. He'd just come in at a steep angle radar off and I'd neither be able to attack him nor get out of the way fast enough. That mk2 does a decent job of supporting others being attacked by planes however. Still, more often than not the plane is just scared off before destroying it.
I think the key factor in all this is fun.
- the plane pilot can attack anything, and if the pilot dies, he gets back into a plane and zooms quickly back into battle
- the heavy tank or assault mech may not be able to attack planes, and if they're destroyed or heavily damaged, it takes a long time to get back into the action
The battle is the fun part, not the trip into the field. Slow vehicles who are constantly forced to make these trips without getting into action because of these aerojocks genuinely feel like they're being griefed on.
So, lets say planes were changed to merely do support damage rather than dropping the wrath of god on ground units...would that really make them less fun to fly? They still can enjoy constant action, something that ground units will never be able to say.
The action is what's fun.