A 1-use-per-day spell system is simply a 14,400 turn cooldown, and can be exploited in exactly the same way as a 600 turn cooldown. But yes, there are better solutions.
Except for the fact that in a per day system your are supposed to sleep to regain your spells. It's not just a mater of time. This way developers can control the ability spam through rest areas placement, and at the same time they are free of consideration about timing.
As for the "save for later" complex, reagents aren't worse than any other limitation. Cooldown wouldn't change anything in this regard.
No, it isn't the same thing. Rare-item reagents say "you can cast this spell up to X times in the entire game, no matter how long you play". Long-cooldown times say "you can only cast this spell X times per game day".
If I cast "Bob's Ragent-requiring Nuke" X times in the game, I'll never be able to use it again - ever. If I cast "Dave's cooldown-requiring Zapper" X times a day, I'll either have to make do with what other spells I have, or wait until the cooldown expires, but I will be able to use it again. That's a clear difference.
Rare reagents just means difficult to find, not limited to a 2 digits number. We are talking of a renewable resource. Just rare, or expansive, or both, not a finite one.
This is a game where you will end up with an inventory full of stuff no matter how magic works.
Sorry, but that is not an good argument for adding even more.
And why more, excuse me? In Ultima, for instance, potions and spells use the same base ingredients. Leaving alone the fact that "you don't want to add more stuff in the game" isn't really an argument here, considering that putting an arbitrary limit to the amount of stuff one carries around in this kind of games is - in short - contrary to their spirit.
