basically because partial def ignore is mostly worthless, u less you stack to a considerably high amount :v
i always tought tho that Expiatio should give both def and mdef ignore, the skill usefullness would increase 100x times already
I had thought so too, but it is a damn shame it isnt. Oh well, at least it means a few hybrid skills like GC and other similar ones can have abit of Hard Piercing from sides of the world.