The problem with Shield Guard is that if the attacker can't see the guard, the guard can't shield the attackers target.
So anytime you get LOS on a target, all you have to do is rotate your model so the guard is out of your front arc. Then the guard can't become target of the attack (since the attacker doesn't have LOS to it) and thus can't take the hit. This stems from the tactical tip on the shield guard model's entry in the books that says "If this model can't become the target of the attack for any reason, it cannot use Shield Guard" (or something really close to that).
This is horribly broken since any smart opponent will never let a shield guard be in the front arc of the attacking model.
The necessary change to the rule is a statement saying the guard needs LOS to the attacker in order to use the ability, not the other way around.