Rarity has, ultimately, NOTHING whatsoever to do with ingame playing power. Look at other TCGs/CCGs - some of the best and deadliest combos might consist solely of common cards. And rare cards might plainly suck in every single situation. They simply are rare and stress the COLLECTIBLE part rather than the GAME part.
Short: Do not try to bind a card's playing power to its rarity. It is not a good path to walk down. [Imagine every good deck woul have to consist of rares only and would be strictly better than all other decks with only uncommons or even commons.]