Exiling cards makes creatures like Soulherder and Hero of Breatgard stronger
A 2/1 for two is already fine, and if Naban, Dean of Iteration draws you two or three extra cards over the course of the game, you've already built yourself a blue version of Dark Confidant, so perhaps there is more potential here than it looks at first glance
With a Turn 2 Otepec Huntmaster, we can curve into a Turn 3 Ranging Raptors plus Rile, which then curves us into Carnage Tyrant with haste
Khans allows you to exile the top card of your library and play it until the end of your turn, while Dragons allows you to deal one damage to any target when one of your creatures leaves the battlefield
The double attack ability that this leader has can be extremely threatening, but without a consistent way to get banish, it just means you accelerate your opponents game plan by giving them more cards
And if we just look at Black & White onwards, regular Mewtwo cards increased by 8.3% while 2-prize Mewtwo cards have increased 35.3%