I always thought it rather defeats the purpose of being able to select your retainers class if it has to be one you have yourself. You can't even level your retainers beyond the level you have for that particular class.

If someone wants to level their retainer as a miner to avoid a bit of grunt work harvesting ore, where's the harm? Similarly, if someone chooses to level a Black Mage retainer when their only job is Paladin, why should SE care? It's not like it suddenly renders markets obsolete. Despite their best efforts, retainers still only farm at a snails pace compared to players.