This would help my entire federation rather than just myself . . . and everyone that is a member of a federation with more than 25 countries as members (which would be a more than a few players). The only people that would be hurt are those without federations. Anyway, I've never seen air defense defend fleets.
As Crafty mentioned, there is always fed branches. As Josias said, you can set up Air Defense wherever you want. Currently, I can set up wings in every country on LU and simply rotate between fed branches, it would take less than a day. It's not terribly difficult, more annoying than anything else. My argument would be (ignoring the war treaty at the moment) that this is entirely unnecessary and pointless. Limiting federation membership, considering that Air Defense wings themselves have their range limited, doesn't make any sense and imposes limitations, weak ones true but ones without purpose, on the growth of federations that prove harmful to less organized federations and annoying for more organized federations.
However, the war treaty changes the equation. Better to have branches with a war treaty option than a superfed without one.