They said they want to streamline the armory system, which I think is a good thing. Having class skills from both main hand and off hand items, while certainly possible, would not be as streamlined.

Now Gladiator will have class skills that involve use of a shield. Probably similar to the existing shield skills, but maybe they will get overhauled.

If the developers want mages to use shields for more than stat placeholders, then they can assign special shield skills to the mage classes. Maybe stoneskin will get a buff if a shield is equipped.

If you want to be a mage who uses a shield as a tanking aid, then you should be required to level gladiator for the cross-class skills if any are made available.