It doesn't have to be some crazy DD its just that with the way Yoshi-P describes what jobs are it is possible that a GLA can become a PLD, DRK, DRG, and THF which in the last 3 cases why not still hold a shield? Unless the abilities would need off hand to be empty or shieldless.
There is a thread about this already but i will sum it up.
Class = Weapon Specialist
Job = Role Specialist