So, you're a GM, what's the hit roll on that attack at the exposed legs from behind? What size shield are you talking about, a buckler? Even a medium shield will cover the arms and head, along with all of the torso. Let alone a tower shield, which would mean your "easy target" would be the ankles? So, here's what I want you to try:
Get a piece of 4x8 plywood, and cut a foot off of the width, so it's 3x8, then cut about 1 1/2 feet off the height. Strap it to someone's back and check out how much target you really have. Maybe an actual visual of what this actually looks like will help you understand what's being presented. As someone who has participated in Live Steel, sword fighting at Ren Faires, I can tell you, you're going to be in for a big surprise. Depending on the height of the person wearing your makeshift tower shield, you may have a couple of inches at the bottom, and you won't see their head, at all. With someone that's really tall, you'll have more of a target below the bottom of that shield, but you still won't see their torso, head or arms. Good luck getting a favorable angle on hitting anything below the shield, instead of hitting the ground behind them. Unless you're going to run up behind them and lay down?
Again, man, it's about simplicity. Based on the logic of, "I have a shield on my back, therefore, I should get +2 AC," we would also be including EVERY item we're carrying as part of our AC. Backpack also protects our back as does the spoons I put in my pockets, or the money bag I have dangling by my hip, plus the scabbards and so on and so forth.
You have to draw the line somewhere. It's simpler to say, unequipped shield, no AC +2, assuming that the +2 is because you are allowed to move it about to deflect enemy attacks wherever that enemy is attempting to hit you.
