9 Crew Crew is a used by Vehicle artifacts that require you to tap creatures equaling a certain power to transform the Vehicle into a creature for the turn
The card has seen a lot of play in competitive decks where it has scored 44.75% wins, according to LimitlessTCG
This card's name becomes "Dark Magician" while in the Monster Zone
They don't have the glaring weakness of requiring a Field Spell and you can have more than one Earthbound Servant/Prisoner monster on the field
I do feel though they may be viable Standard and Extended options, but because of the extraordinary power-level of Extended right now, I think that perhaps there will be too many unfavorable match-ups, especially the modified Elfball, numerous artifact combo decks, and maybe Zoo, but if the deck can be modified to a state where it is a tenable Extended option, that's terrific in my opinion
I envision using Elemental Mastery, Freed from the Real, and Dragon Throne of Tarkir to really assert my dominance