House Rule Rebuttal: Back Shields
Well, I agree that getting +2 bonus (or in some cases perhaps more) vs flanking attacks seems like a relatively big deal, at least anytime you are being flanked. I agree that mechanically, it makes more sense not to split precision damage from the flanking bonus, but, admittedly one could argue someone can stab around the sheild. However, that is also very true for the original attacks too. One could also point out that a full plate, will basically have a sheet of steel behind it, why doesn't simply wearing heavy armor aleviate the flanking bonus? (basically, the bonus is not being able to be aware of what is going on all the way around you)
Asuming you are talking about a heavy sheild, giving a +2 AC vs flanking attack, means you have an absolutely fully effective shield being actively projected out your back, as it is getting a full benefit. I'd be more inclined to believe it would have to grant 'less' than full bonus, meaning none, for a light or buckler (since they are normally only 1) and perhaps a +1 bonus with a heavy sheild on ones back. Keep in mind, a flanking attack need not strike them in their back, it can just as easily hit them in the leg, arms, or head, just like any other attack. It just is easier to land some sort of hit, as the person is having to shift their awareness around to try to keep track of their opponents.
Or, if they insist that they should get their bonus, and you decide to let them have it. Don't forget, if they are effectively 'back-wielding' a shield, they should be inhibited by wearing it. This means applying the shields armor check penalty.
Next point out, unless they have some sort of feat granting them EXTRA training to effectively use a 'back sheild' with expertise, they are non-proficient with it. That of course means they well take a penalty equal to the shield's Armor Check Penalty to all attack rolls, STR, and DEX based skill checks.
It could be amusing to consider someone suggesting they are going to strap a tower sheild to there back and battle that way, getting a benefit from it. Sure, you might manage to negate a couple lost AC points because of it, but you'll be taking a -10 to hit and skill roll!
If they want to argue the sheild makes their back more armored, you could track down the rules for piecemeal armor and let them 'effectively' upgrade their 'back' to something like Plate when they are wearing a steel shield on their back.