You are misunderstanding the way 4E attack powers work. If you look at monsters' defenses, you'll see that the AC defense is typically ~2-3 points higher than Fortitude, Reflex, and Will. The 4E designers purposely did this to balance weapons' proficiency bonuses. In other words, an attack power using an implement instead of a weapon (which doesn't get the weapon proficiency bonus) virtually always targets Reflex, Fortitude, or Will instead of AC. The chance to hit is the same for an implement-based attack power targeting F, R, or W is the same as a weapon-based attack power targeting AC.