I agree that going to the level of WoW mounts is a little ridiculous but that's the other extreme. The first being only having ONE creature to ride. Again I ask can someone give a legitimate argument for the exclusion of anything else but Chocobos aside from simply being afraid of change?

The series is constantly moving forward and adding things that weren't seen in previous games. This game as an MMO could greatly benefit from a variety of different ridable creatures that would allow people a greater degree of customization based on their personality or play style. I think restricting it to things that are logically ridable is fine (no Bombs, Wights, Cactuar, Gnats etc...). Though things like Drakes, Goats, Crabs or Dodos (Dodobo!) would make sense as they're big enough and logically one could ride on them. If they're going to add customization in terms of armor and accessories to Chocobos let us have it for all of them!