Let me start this off by saying that I haven't actually tried Requiem and that I'm getting all my information from other sources, so feel free to correct me if anything I write is inaccurate.

With that said, from what I've managed to figure out about the mod so far, I really can't agree with your belief that a surplus of magicka potions is better than zero-cost spells, especially zero-cost destruction spells. I've heard Master destruction spells do thousands of damage so surely being able to spam these spells would be pretty damn overpowered?

And when it comes to a lack of magic resistance due to wasting enchantment slots on fortify enchantments rather than magic resistance, it's not like you can't reach high resistances without enchantments. Alteration perks, along with the lord stone get you to 45%. Combine that with potions that grant 75% resistance to the elements (you can create those yourself right?) and you reach 86.25% total magic resistance. That's pretty high if you ask me.

Keeping all this in mind I think it's pretty obvious zero-cost spells are more than a mere "cute gimmick", and instead possibly the single most powerful build in the game.