An airbase also acts as railroad, farmland if the base has supermarket, irrigation if not, and adds one food to mountains and three to hills - all you have to add is the mine. I eventually always add the developments, not just because OCD tendencies, but unit pathfinding doesn't recognize it doesn't need the roads and takes stupid paths.
-It also acts as plane repellent later, to have all those airbases on your land. The AI avoids those squares putting their plane to sleep vulnerable until next turn.
Now, since I never heard of this with airbases -in posts from 1998 at that- I assume this is something, probably inadvertent, introduced in Multiplayer Gold, or Fox Ahead's patch. Can anyone shed any light on the subject?