I noticed a serious problem with the TerrainDefenseForAir feature as it pertains to fighters located in cities. When I disable the terrain defensive bonus for air units, fighters no longer receive the special "scramble" bonus when defending a city against bombers. Instead of having their defensive strength quadrupled, fighters merely have their defense doubled against attacking bombers. Furthermore, fighters receive no bonus at all when defending a city against enemy fighters. (Ordinarily, their defensive strength should be doubled against attacking fighters.) So if you ever get around to reading this, TheNamelessOne, you should be aware that disabling the terrain defensive bonus has the inadvertent effect of overriding the fighter "scramble" bonuses, which are so essential for balanced game play.
By the way, I'm using Version 0.13 of the patch, and rigorously tested air combat on a city square that has no defensive terrain bonus (i.e., grassland).