Just my opinion here, but.......If my opponent is using non-transport aircraft he is doing me a favor. They have just spent points on a formation that cannot sieze an objective, hold an objective, or contest an objective. Basically one less formation I have to really worry about tackling on the battlefield.
Any hits they score on my ground pounders would be the same as they would suffer had my opponent had artillery in place of the aircraft.
Rule:4.2.4 Flak Attacks page 50 Epic-A rule book 2nd paragraph, third sentence
"What's more, a flak weapon can make any number of attacks per turn against different aircraft formations, as long as it does not attack the same aircraft formation more then once a turn."
Plop down the points for two Hunters and put them in formations that will be on your flanks, or be really mean and get three

Now every single enemy aircraft formation that flies onto the table is gonna get shot at at least once, twice if they are trying to go after something really important.
Well that's my two cents on aircraft anyways.
Exceptions: Ork Fighta Bommas and Dark Eldar fighters ARE useful to those armies because they lack any long ranged arty, and the ability to hit the enemy deep in his rear areas, is nice to have. Despite this I still do not field aircraft when I lead my Dark Eldar and Orks into battle.
Heck no pilot's brain functions effectively below 10,000 feet.
Jaldon