Evoker benefits the most from the extra power level. Offensive spells PL will scale damage, bounces/projectiles, accuracy, penetration, and duration (if applicable). Evocation has offensive spells at every level, some of the most used spells for a Wizard. The Evoker special bonus gives a chance for extra damage. Many other spells just get a little extra duration (or accuracy if a debuff), which doesn't mean much. And the special bonuses are much weaker in other subclasses.
I feel like Evoker is decently balanced compared to generalist. I don't really like losing two schools though, it takes away so many good spells. I wish they'd choose some other penalty, other classes don't lose 2/5 of their ability tree from picking a subclass.