I think a better term is "location based damage".
This, as without a hitbox you can't hit your opponent at all. Oblivion simply uses just one
Either way, if people decide not to wear a helmet because they're playing the game as a dress-up one, it's a risk they take. Heck, bind your helmet to a hotkey and take it off when you're not in combat?
Also, @WerewolfNeverine:
This tech only works to a certain degree. It won't detect if an arrow actually hits armor or not, but it'll detect which limb is being hit and
then the game checks the presence of armor for the respective limb.
Therefore, an open helm could still be better than a closed one, depending on its stats.
Still, it makes sense if closed ones protect you better, right