When it comes to collectible games, not all the cards can be equally good. If they were, it would continually up the power level of the game as well as remove the strategic aspect of creating a deck - one of the main draws of the genre. It's also not economical, either for the manufacturer or the players, for only rare cards to be good. In order to hook and maintain the audience, some of the cards must be less valuable or useful.
Good cards are spread across all rarities as much as possible. Since the criteria for acceptance as a rare isn't just the card's usefulness, this will naturally lead to some rare cards being useless, or at least so specialized that they are difficult to use effectively.