My view is that the bracers of armor are balanced around the cost of increasing the AC bonus on armors. This makes sense to me since each class is designed to have access to certain items, and abilities. The classes are balance around these restrictions. A fighter not being able to cast spells is a restriction on his capabilities, just as a wizard/monks inability to use armor. Thus by giving more armor for less cost for bracers it throws off this balance between the classes, since it would allow the non-armored classes a faster AC progression. Saying that it is ok to have this imbalance in items since class X is gimpy does not justify an imbalance in items. If class X is gimpy then change class X not an item that is usable by anybody. Even if you can only forsee class X using the item at the moment, there is no telling what the future will hold, and I personally prefer to use rule 0 as evenly and rarely as possible. (since if it becomes broken in the future you have to reverse course on the ruling)