It keeps the spell lists clean and makes it a tactical decision to delay or even deliberately avoid learning certain upgrades. Part of the game's thinking/optimizing your way to victory theme is knowing when to hold off on something you don't need.
Aamon, Moloch, and Kerberos are all considerably weaker than Belphegor, and absorbing the leftovers of their domains gives Aura more mana to tackle Mountainreach if the materials, gold, and skill books weren't reason enough to clear the non-Sathanas portions of Draknor first.
Belphegor's entire gambit is that Aura will need at least nine days to clear out the barrier fragments and confront him. It's supposed to be a resource-intensive process where miscalculating or being too lazy to prepare enough mana forces you to end the day. Also creates another use case for Aura blessing her own energy potions.
Overall, it feeling bad is healthy for the game. Same with fire II and bomb+.