It's definitely not a bad card. If it's something you think your playgroup will maindeck often, I'd give it a go and see how it does for you. But rant's right in that Gruul is pretty deep, and while Command can be a good support card, RG does have some unique role-players you'd be passing on to get it in there. In a cube big enough to have 7 cards per guild, I would think that Kird Ape would be pretty important for your 1-drop saturation.
The thing about Ape is that it does exactly one thing, where Atarka's Command does many things. If your Cube doesn't need generic 2-power 1-drops to support aggro, you're much more likely to have slots for more interesting cards.
EDIT: Of course Command still has to compete with the completely glutted 4-drop RG slot, but if you don't want to run all 4-drops in your Gruul section, this is one of the coolest non-4 Gruul cards and definitely worth considering. Especially with the new landfall stuff and new-ish army-making stuff in RG.
I agree that CC of good RG cube cards could somehow force you to look elsewhere, but I think I'd prefer to run stuff like Burning-Tree Emissary, Ghor-Clan Rampager or even Domri Rade as a none 4-drop Gruul card rather than play the Command. And like others said, Ape is still a great option. Even shatter creatures like Viridian Renegades or Tin Street Hooligan could fill the hole.
Cheers,
rant
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EDIT: Of course Command still has to compete with the completely glutted 4-drop RG slot, but if you don't want to run all 4-drops in your Gruul section, this is one of the coolest non-4 Gruul cards and definitely worth considering. Especially with the new landfall stuff and new-ish army-making stuff in RG.
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