Starcraft uses the flying to seperate land vs flying creatures. Land creatures cant fight flying (except by shooting) so the only distinction is to make it clear that they fly (pretty easily done through the use of shadows).

Starcraft height levels are a bit tricky. If you look at the screen shots there is a clear view of multiple heights in the game yet once you understand the flying thiing (being nothing more than shadows) and then look at the games it can be seen that all the levels are actually only made by the tile look and feel and the fact that the units cant cross the level heights. (You can also see this by watching a flying unit travelling the sides of the height levels and the flying unit never chnages its trajectory - indicating a single level of tiles).

By the way - you can also see this in WarCraft. I havn't played WarCraft 3 much but I think that the height views of it are also just tilebased.

Starcraft is the first and last game that I played through all the scenarios. :shock: