Is there any reason to have more than one air marker in a space?
I pretty sure that air superiority in a combat can only be used once and that's the first time there is a tie. Planned Bridges also only require 1 air marker regardless of the number of artillery and it can be ready or flown so again multiple planes wouldn't matter.
Multiple air superiority markers in an area would allow you to win more ties, so provides an additional defense.
Per rule, if a ready air marker is present, a tie will result in it being flipped to flown. A second ready air marker would be available should another tie occur, and so on.
Though only taking effect on a tie doesn't seem as useful as a straight modifier, it's essentially a +1 on defense only used up when it's needed (as opposed to some markers that are used up when applied, whether it made a difference or not), and can slow down the momentum of an assault by stopping/preventing the Big Push bonus. Two or three air markers in an area can be better than some other markers whose modifiers might not have been needed because you rolled high anyway.