Im with the op all the way on this, incap system is a very interesting one, if they could expand more that would be awesome, it would surely add a lot of strategy, but they need to add reward according to the difficulty of disabling enemies, of course the main reward on this is to have the boss or mob being unable to do certain actions, i liked how the horns of the elder buffalo gave the mats to make a really good lance when it was released, how you needed to farm certain items from the goblin NM's to get a really good sword in that time.

They could even add more odds of getting drops by the number of parts you disable during the fight, this will definitely make parties more balanced and will add strategy to every boss fight, even for farming, just check the old chart of weapon skills, some classes had exclusive ws focused on certain parts.