Because even if he has such a stunt he still has to pay for the magic with mental stress and use up his next action; resource-wise, nothing changes.
Stunts like that (Riposte and the like) are good when you quickly want to finish an opponent, not for defense. In fact, they are bad for defense; by using up your next action, you cannot move, even though normally you might be able to move with supplementals or speed powers. And mobility is paramount in most combats.
This is exactly why we aren't looking at is as a stunt. Using Riposte seemed like a good idea at first, but it just didn't seem to fit the bill for the element he was going for.
Essentially, the player mentioned the idea of having an aura around him that would deal damage to those that struck him, but it offered no defensive bonus. To me, it seemed that having this "aura" up with Kinetic energy seemed more fitting than a Ward, especially since the shifts of power that would normally be a block are now equivalent to an attack roll (i.e. in the example I posted, the attacker has to do a dodge against a Weapon: 0 attack worth five shifts).
Not certain if this concept for a spell is balanced, which is why I mentioned it.
I'll keep the Wards and Warded Items ideas in mind, because they are still rather intriguing, and may be VERY useful should I start using the general idea of Sponsored Magic: Runes for ward-like items. Otherwise, keep the ideas coming! Thanks again for the input!
--Crion