Because it would be a passive representation of the actions and skills of your character - that is what "game play mechanics" usually means. They need a whay to measure your character abilities in numbers, this is one of the ways of doing that..
I think their should be a seperate system entirely to not confuse defense with dodging.
Like......the magic failures you had in Morrowind can be similary implemented for melee and ranged attacks against unarmed fighting opponenets.