They have to fit it with the storyline in the ul'dah starting you saw how crazy the goobue got it be dumb if you can just use it as a mount without some backbone in the storyline would feel weird and not right within the game.
hope SE come up with a great reason why there other mount now.
If they were able to come with reasons to implement swimsuits and pumpkins in the game, I'm pretty sure riding a mob seems like a piece of cake to them.