It was perfectly logical for two reasons: 1) starting with the first of the weakest Numbers (Leviathan Dragon) alongside introducing Xyz Monsters as a whole would've made the latter group seem useless by comparison, especially with all of the more-miss-than-hit effects of Xyz Monsters before they became more practical
This makes them incredibly good, especially when most blue wheels either exile the graveyard or return the graveyard to your library
Other considerations are: Refuse / Cooperate, Flare of Duplication, Return the Favor
Im often harping about how the problem isnt that white and red lack for card advantage , its that they lack for card draw
If you have both Monster Manual and Kona on the battlefield, you will be able to get two expensive creatures for little to no mana investment
Go for cards that show up in a multitude of decks so that you can switch if youre not happy with your choice