The events of Cosmo Canyon are not important outside of introducing Bugenhagen; and the plot could have easily been rewritten to have Barret do that since he has connections there. Red XIII's whole point of going is cause Grandpa told him to. After that he doesn't really have another story scene until you get his ultimate weapon.

On topic though, I don't really find the XII cast underdeveloped but that's my own opinion so we can all just leave it at that if we wish. I just find it funny that people act like Penelo has to have some world shattering backstory to be justified being there. To be honest, once Vaan learned the truth about his brother and rescued Penelo; he probably should have been left behind in Rabanastre since his personal issue is sorta resolved by that point. Lady Ashe is pretty much the main character from a writing standpoint.