GS is not THAT ineffective : you will not kill tons of enemies, but you will disrupt quite a lot (esp. if your GS in unopposed). It translates into forces that don't participate this precise battle, which in turn help a lot in the final CV calculations. As the Allies, I use twin bombers to do that, it's working quite well
On the other hand, the GA on a single hex can be useful, but remember that the air battle reports LIE like my ex-wife. Actually, you don't kill a lot of people. Although I agree it may be enough to prepare for a breakthrough (in Italy for example).
Of course, if you do it a lot, with all your air forces on all the front lines like someone did to Pelton quite a while ago, it's significant (like 10k per turn). But then you are neglecting other aspects of the air war such as Strat Bombing, Rail bombing, Airfield bombing (yes, it's important too), and attrition warfare is not really the way to win IMO.