Other than producing units that require a certain level of Air Base, what positive effect is gained with higher level Air bases? I see the negative effect is higher upkeep cost, but I see nothing positive. I could see creating an air corridor with high level air bases that would refuel planes quicker, but I don't see anything like that in the description notes of the Air Base.
"For what shall it profit a man, if he gain the whole world, and suffer the loss of his soul?" -