This mostly seems like a good way to encore a bunch of your stuff back after a board wipe to win the game, and as such I think it's a lot better as a surprise from the 99 than sitting in your command zone
This is chaotic, as you can get practically any spell from your deck, be it one of the biggest, baddest of the Eldrazi titans or a Llanowar Elves
While Deoxys' other rival, Jirachi, boasts a roster of valuable cards, neither Pokemon can compare to the sky-high prices Rayquaza's best cards can reach
This legendary creature provides an exceptional balance of offensive and defensive utility for its mana cost
In practice, this doesnt seem to be much of an issue due to the redundancy built into the deck, and it works out that the decks fail rate of losing to itself due to mulliganing into oblivion is not much worse than the Legacy version
Tutor abilities, especially once that put cards on the battlefield, are just endlessly useful