Sky Island support cards trigger at 6 or fewer total DON, creating a self-reinforcing loop: stay lean, hit hard, repeat
Each of these effects is well costed on its own, so even if you can only pick one, 5-7 mana will usually ramp out your commander or another expensive spell, and effectively drawing 3 cards will usually get you to a land drop or something you can play later in the game
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This includes Holos, Reverse holos, rares and Ultra rares
Instead, it was banned in Pokmon Trading Card Game Online
Blue alone is well known for having no shortage of solid cantripping creatures, and so there's plenty of ways to draw cards