I believe Blizzard's original reasoning was that they wanted to annoy a forum poster from the future by the name of clbembry.
If I remember correctly, they managed to gleam a vision of the future where he could possibly be annoyed by the introduction
of a flying mount to few the various panorama's of the world.
In all seriousness, they were introduced for multiple reasons.
1) The design style of outlands was (and is) very disjointed, it's a cracked planet at the end of the day, and not overly accessible
2) They were brought in as one of the "key new features" with TBC, designed to be one of the many hooks to bring players into WoW
3) The vista effect and freedom of movement, flying in the air, is a pretty awesome way to see the game world you create.
off the top of my head at least (: