Spawning one copy is arguably enough, doubling the duration of protection from 6 dims or 6 dials.
Your comparison to earthquake is legitimate, but it's harder to draw 2 out of 6 Earthquakes than it is to draw 1+1 out of 6+6 Whirlwind and Nightmare, which means this combo, although a duo, can probably be gotten out faster. That, and Whirlwind has versatility outside of killing pillars, while Earthquake counts as dead cards in the face of a pillarless. Just food for thought.