To further illustrate Kuro's idea, let's take an individual case: Paladin

http://img828.imageshack.us/img828/9...nchcasepal.png

As you can see from this flow of character development. Paladin builds off Gladiator and is an evolution of the base class. You still have full access to all the things you've learned as a Gladiator, but you also now have access to actions specific to Paladin and more advanced versions of actions that you learned as a Gladiator. So you can still mix and match actions up until a certain point. But as you continue down the path of Paladin, you surpass being just a mere Gladiator and now wield your sword and shield with purpose. To protect your friends and citizens of Eorzea. As you level up you can eventually equip Paladin specific armor and look all spiffy.