The main reasons for choosing Air Magic are Fly, Dimension Door and Haste spells. The first two are perhaps the most powerful adventure spells, together with Town Portal. However, both of them are typically banned in custom maps. Nevertheless, Expert Air Magic causes Haste to be cast as a mass spell, which still makes it worth the secondary skill slot. Other noteworthy spells are Lightning Bolt and Chain Lightning, both powerful combat spells. (unsigned user)
Don't forget view air, it lets you see ALL towns and player heroes even in areas that you haven't explored yet. Requires no movement points and only 1 spell point.