That isn't necessarily a benefit for the fighter. Inability to attack defenses other than AC isn't an advantage.
Sure, but the ability to attack other defenses is usually offered as an even-up equalizer to the weapons-based classes' access to weapon proficiency bonuses to-hit.
My point is that the proficiency bonuses have no downside, whereas targetting a low defense may mean using a less effective power.
For anything other than a pure damage-oriented War Wizard, the essence of the Wizard class is to lay status effects on the enemy, and preferably more than just one. And not just *any* status effect: it does little good in most cases to slow or immobilize Artillery or Brutes who have reached the front line.
But the Wizard and the other non-weapon-oriented class builds are likely to find that the perfect status effect for the situation is highly risky because it attacks the target's best defense. High reward, but high risk.
Truthfully, we're talking about relatively small differences here. If the Wizard has a 40% chance of hitting one defense and a 50% chance against another, the result will usually be the same.
Anyway, I think that's about all there is to say for my side of this discussion, without going into one of those spreadsheet-based defenses that pit every class and power against every monster, cross-referenced with the Book of Kells and the writings of Nostradamus.