I wouldn't be sorry to see them in game. I love mounts.
That said, have some faith in the developers' visions. They might just have some inkling of how to make a game. Just a hunch
Jokes aside, I'll say this: it's a lot easier to implement something later, than having it to begin with then taking it out.
Off topic: I bet there are times when Blizzard regrets adding flying mounts. But then they count all the $$$ they made from selling sparkle ponies and sparkle lions and sparkle dragons and they feel better!