Having this spellbook (or the prospect of Cthulhu obtaining this spellbook) creates forced behaviors by your opponents. The will be reluctant to leave a single cultist in an area with a gate even if you never take the action.
While the power cost for this spellbook action is high (3 power), the addition of your cultist to an area where they can potentially control a gate while simultaneously removing your enemy's cultist and controlled gate is a big power swing.
Remember that this spellbook can't be used if you don't have any cultists in your pool. Devolve one sometime before you want to use Dreams to return the cultist to your pool.