Who cares if it's a weapon or armor or a fish. You put it in your offhand, it's a class, it levels up along side your main weapon's class and is not meant to exist independantly.

It is a side dish. It accompanies the main dish. It serves no purpose existing on its own but together with the main dish serves to enhance it. Also you can take it or leave it. Why you would leave it when it simply requires wearing the shield and forgetting about it is beyond me. You never know when some of the abilities it grants you might come in handy.

What the hell would a shield class or job be anyhow? A combat dummy?