Because some classes should have certain skills. Rogue should have Thievery. Wizard should have Arcana. Paladin or cleric? You should have Religion. Going Paladin/cleric gets hosed RAW, okay, but do you really want a MC Wizard without training in Arcana? That's even worse. So go ahead and treat it as Skill Focus, in a home game. That seems fair to me. But understand why a Wizard should get training in Arcana, and you'll understand why a Cleric should get training in Religion.Why the feat doesn't just let you take a skill from the other class' skill list is beyond me,