I still think that any bonus to a defense roll should come from a stunt, just like would be necessary with every other piece of equipment covered by the game. And I think that tower shields could definitely be used as weapons--they're heavier, and getting bashed with one is going to hurt. Watch the fight scenes in 300 or Troy--those are comparable in size to, and probably heavier than, modern riot shields, and they're used effectively to smack people around.
Functionally, having it be Armor:1 with a +1 to defense rolls isn't that different from having it be Armor:2--on a failed defense, it still amounts to 2 less shifts of stress you're taking. I see it as Armor:1 would be things like bucklers, Armor:2 would be about the size of a trash can lid (or Captain America's shield), Armor:3 would be a tower shield and larger.
Shields have been a common piece of equipment and weaponry since human beings first conceived of the notion that it would be better to not take a club to the skull, so I don't see why they should be treated in a unique way when the rules already have written-in ways to take care of bonuses and penalties. To me, it just doesn't make sense for a skill roll to be effected by a non-magical piece of equipment without a stunt. The game text makes it clear that if you want to modify a skill roll, compels, powers, and stunts are how you do it.
As for it being a full equipment sent in a single item...Well, yes, a shield is a versatile item, and has been for the last 10,000 years that human beings have been using them (though I would advocate giving it a lower Weapon rating than its Armor rating to emphasize that it's a primarily defensive item). That's more or less the entire point of using one in the first place.
But in the modern day setting of The Dresden Files, you can't carry them everywhere without getting unwanted attention, so there's a built in limitation, the same as with any weapon: You only get the benefit if you can believably have it with you.
Use Compels to say, "Okay, you're about to investigate a Red Court nightclub--here's a fate point, you realize you're not going to be able to bring that 2-foot-wide Viking shield in with you," or use a Compel to say, "The battle's wearing on, and that shield of yours isn't getting any lighter, take a fate point and you'll start taking penalties to your defense."
For bonuses to skill rolls, have a stunt like, say, "Shield Master: Through training, you've learned how to use a shield more effectively. Add +1 to your defense rolls while wielding a shield, and +1 to maneuvers made while using the shield." or "Stone Wall: With a shield in your hand, your defense is damn near impenetrable, but you lose some momentum on offense. Add +3 to skill rolls to defend when using the shield, at a -1 penalty to movement and attack rolls."