Well, I don't know if you have practiced martial arts, OP, but maybe punching someone in face is more painfull, but I challenge you to guard yourself against someone striking you from behind when your attention si focused on someone attacking you from the front
In my opinion , this is more this kind of situation Monks in FF try to simulate rather than just "it hurts more in the face". When your opponent expose his back, he is far more vulnerable than when facing you, giving you a lot more opportunities to land a hit. If you manage to bypass a foe's guard and reach his flanks or back, you have the advantage ^^
Besides I think it is pretty well emulated with monks here as you constantly move to be at the best spot to hit. That inclues taking risks, but you are rewarded for it.
Same as PVP, if you manage to stun or find a way to get in the back of an opponent, he will be easier to hit, either because you will hit harder, but also because his evasion is reduced from behind.