On a Celestial Master it's like a free Power of the Spheres during battle. Which is very nice unless you like casting Geyser or Acid Bolt his entire career.

A Power of the Spheres and a Crystal Shield on a Master makes him able to cast nifty spells like Falling Fires, Falling Frost, Orb Lightning (effectively), and when he gets low on fatigue, Smite a billion times.

Unfortunately, the Computer doesn't say "Look we're at 99 Fatigue, I am only going to cast Holy spells" it says "Lets cast the most fatiguing spell I have"