I think you may be ignoring the fact that striking an object specifically states that only deflection bonuses count for objects being held by the opponent.
And what if I tried to disarm the wand instead. According to diarming rules I just have to roll higher than the opponent to disarm - there is nothing about AC in that whole option. And since I am disarming a wand, it is the weapon of the opponent for disarming rolls. Shield (or any AC bonus for that matter) does not help at all.
Would you count the AC from the shield if instead of hitting the wand, an attempt to grapple was made. The combatant is coming straight on, the shield is in the way, shouldn't that give protection? It would if it was a deflection bonus, however it's not a deflection bonus, just a cover bonus. And so doesn't count for grappling.
What if I am a spell caster with either a ranged touch attack or melee touch attack spell. I'm right in front of the shield using defender, they have the shield facing me. Why would the shield not count there? Using your theory it should. The attack has to go through the shield does it not.
I know that grappling, and spell casting are touch attacks, however the point is made. The shield spell only protects the caster's physical body, not objects carried, and only against non-touch attacks.