From: Winnipeg, MB
Forts certainly help save troops from bombings/bombardments, no doubt about that. The effect depends on the incoming caliber. CL/DD bombardment will do nothing against high forts, but BB will kill some stuff even against level 9. On the other hand, have a 0 forts enemy bombarded with CL/DD TFs and you will be pleasantly surprised. I'm doing it right now against the Chinese respawn in Foochow (the last beta has them reviving in any large enough base in the AI game)
Forts should not save your planes like they do troops - planes are too big to hide in trenches. Not to mention ships and runways. Yes, airfield earthworks were used in WWII, but I assume their defensive effect is limited, no caves for planes to be built. And if the effect is in any way in game, it is also limited.
It would be reasonable if the airbase hits (that destroy supply) were somewhat negated with higher forts. You can hide your supply in your trenches. I don't know if this is the case in game
I think forts would also include any camouflage efforts. Aircraft in revetments off in the bush could easily be hidden under camouflage. The taxiway itself would be harder to hide.
No matter how bad a situation is, you can always make it worse. - Chris Hadfield : An Astronaut's Guide To Life On Earth