The problem is that "weapons" skill would probably apply to shield use as well...
And it seems a bit unfair to make someone use up 1 refresh just to be able to use a shield.
Well, if they just want to be a sword and board fighter they can defend with their Weapons skill just like anyone else.
On the other hand, I would permit anyone with a shield to add +1 to their weapons defenses in the subsequent round as a supplemental action, no stunt.
If they take Shield Training (or whatever you want to call it) I'd make readying the shield a free action, so they'd get +1 to weapons defense whenever they have a shield in hand.