Sorry for the OT, but I simply have to answer this one:
Flying units have several advantages over walking units, ever sat on a building (in X-com that is) where no Sectoid could reach you? All you have to do is to walk to the edge, fire, walk back, be safe. And when flying the chance of getting hit by a grenade becomes rather low, too, albeit I had a floater killing one of my units in flight with a grenade recently
I don't say the sectoids are helpless, BUT having flying units against nonflying units really makes things easier. It isn't an autowin situation (except with Chryssalids, where it is) and you suffer penalties in accuracy if you hang in mid-air, BUT the mobility/position advantage is easily worth it.
Oh and there is a mod that adds those cover aliens, see for the thread with the name COVER ALIEN.
Back On-Topic: I really like the new Battle Tank, btw, especially that you balanced it and did not make it the only tank to have. Good work.