Our goal from day 1 of CardMill has always been the protection of cards
This kind of effect is always popular amongst players, and getting to cast such a spell twice--once for free during your upkeep via Rebound - is sure to be a hit
Sheldon Menery, the godfather of Commander and head of the RC, even said that the card is too generically good and pleaded with Wizards to Please never print this card (Wizards apparently asks the RC for input during card design)
And in the latter half of Khan of Tarkir, let's evolve into Abzan Control, a deck that has further strengthened control by adding a mass in decline to the Abzan midrange
While there's no protection for your attacking creatures, the potential benefit from other player's attacks is incredible
This means if two players both select the Mewtwo booster, they will find themselves earning the same five cards