In a deck where Lorehold, the Historian turns your massive endgame spells into (2)-mana Miracles, being able to stack your deck at any time is game-breaking
Repudiate / Replicate One of the good things about split cards is allowing the user to play narrow effects, and fewer cards are narrower than Stifle (Repudiate)
While the addition of cards like Mage Power and United We Stand would mean she couldn't always attack over common boss monsters like Summoned Skull, Injection Fairy Lily was good enough at her job that she was banned for a format to allow players to summon their best monsters without worrying they would be attacked over by a tiny fairy on the following turn
For those who aren't subscribers, you're not missing a card that's going to change the game's meta
The problem with this strategy is that it means the very first card you see is the code card
People love making their token piles, whether its treasures, creatures, or copies