Light Shields and Bucklers come standard with a -1 armor check penalty that applies to lots of stuff if you are not proficient with them (namely, any skill check involving moving and attack rolls). However, making a shield masterwork drops that penalty by 1, making the penalty 0.
This effectively means any character can use a masterwork buckler (or light shield, if so inclined) with no penalty whatsoever, other than the weight and arcane spell failure (if they cast arcane spells). Rouges, and other classes that typically wield one-handed weapons (or even spellcasters that choose to make such a shield out of mithral) can use such items, despite being not proficient, and with no penalties to boot!
Therefore, I propose a minor change to the wording of Masterwork on Armor and Shields: Namely, simply stipulating that any reduction in armor class from being masterwork (and by extension, mithral and darkwood, which must always be masterwork, I believe) is only gained if you are proficient with that shield.
Thoughts?
EDIT: For wrongness.
Darkwood does not lower ASF.
My mistake. I will edit the rest of it.
The ACP applies whether you are proficient or not when it comes to skills. It only applies to attack rolls when you are not proficient.
I don't agree. The idea with the extra cost of special material and masterwork is to make it so light and well made that the ACP does not exist. Sure they are not spending a feat, but those special materials are rare, and expensive. It is not like it is free. I wouldn't take a feat just to drop the ACP. It is not worth it. I would just keep going with my miss chances to not get hit, and I would take the money that would have been spent on the shield to buy more scrolls or some other magic device that makes me better.