ORIGINAL: Saturn V
ORIGINAL: Steely Glint
There's a good reason bad terrain generates defensive bonuses.
For ground assaults by infantry and armour, that makes sense. But an airfield requires large, flat areas for runways, taxiways, revetments, hangars, and an array of operations buildings. Indeed, it is difficult to locate an airbase on bad terrain because of the engineering that has to be done to prepare that terrain for the requirements of an airbase. And all that flat ground needed makes an airbase easy enough to spot from the air.
So airfields should not be gaining a defensive benefit from bad terrain when attacked from the air.
(If it's a land assault, then the defensive benefit makes sense as it would indicate the terrain surrounding the airfield which the ground forces have to move through to assault it.)
I hate to bust your bubble but I can tell you that as a pilot it can be extremely hard to find an airfield even when you need to. It was even harder during WWII. Plus none of these airfields were lit up at night nor had visible landing lights. I've had to find airports with three+ major runways after their normal operational hours (i.e. all lights were turned off) and it was not easy at all. Especially when you forgot the com code to turn on the runway lights
So the terrain modifier for air attacks could simple be the 'cost' in fuel/airtime for attacking aircraft to actually locate the airfield. Also local weather, haze, etc. all play a part so I see no reason to change this at all.