I think people are underestimating exactly what gaining 15 life means. Like Rasmus said, that's a whole lot of time you are buying. If your deck's card quality is better then your opponents, you probably can't lose are resolving it if they don't have a lethal engine on the table. Lots of decks throw away cards to get in damage (burn, early attackers that get outclassed). Mystic Confluence undoes all that, making it likely to be the equivalent of undoing ~4 cards the opponent SPENT MANA ON in many cases. And unlike more narrow cards, when you don't need life you can just spam tokens or blow up enchantments. If anyone has run primal command they know that gain 7 life is a super strong mode on that card. I won't run the card because it's powerful in a direction I don't want to encourage, but it sure is powerful.