From the "realism" perspective, you want your aircraft as close to the front (or their targets, whether offensive or defensive (e.g., air defense)), as is secure. IF that were somehow applied, you would have an airbase "unit" that could EITHER base aircraft OR move to another location. Like Civ4 spies, it would not be able to serve as a base in the turn that it moved. Yes, I know the odds of this occurring in Civ5 are VERY slim.
In WWII European theater, Allied bombers & fighters flew from England because they HAD to. After the Normandy breakout and the supply pipelines were in place, many bases were moved to France. In North Africa, similar circumstances applied, even more so.