Reading through the forum and from personal experience, there is not too much point worrying about airplanes in the game currently. Then I read something today that could make them more useful.
In Barbara Tuchman's The Guns of August, she quotes the French Grand Quartier General as saying, after the Battle of the Frontiers in 1914, "Finally, we must copy the enemy in using airplanes to prepare artillery attacks" (p. 261). Brilliant! Why not give artillery a +10% bonus from available airplanes? This would be similar to the +10% bonus infantry gets from tanks.
Adding the reasonable idea that airplanes can reach two regions away, here is the idea fleshed out a bit. If artillery is involved in a battle within two regions of an airbase, airplanes will attempt to help the artillery with recon of the enemy positions in that battle before the battle occurs. If enemy planes are around, there may be an aerial battle to prevent this assistance. If not, or if the friendly side wins the aerial battle, the friendly artillery gets the 10% bonus in the battle. This would make airplanes something to care about and much, much more useful than the little bombing they sometimes do currently.
In addition to this I would also combine a couple of forum suggestions already given: have airplanes give +1 detect all the time to all regions within two regions away. That would also help make them more valuable.