Ah, but see the idea is to gain arial superiority (in keeping with Rommel's historical campaign). The idea behind the map design is that aircraft are too busy dog-fighting to have time to bombard ground targets. If you gain arial superiority (i.e. enemy has no aircraft in the air but you do), the map is yours because you are then able to support your tanks.
And yes, the additional role infantry play is defensive (manning AA guns, for instance, is unique to infantry).
And yes, the additional role infantry play is defensive (manning AA guns, for instance, is unique to infantry).