[Skarg]

The main thing to consider about using IQ for spell fatigue is how it throws off the balance of wizard characters, whose main use for ST is to power spells. (Yes, it helps them survive injuries too, but the best way to survive injuries is to avoid them altogether.)

Particularly if you are doing _Wizard_-like arena duels, or are otherwise interested in the trade-offs of different types of wizard characters, then shifting more functions to IQ tends to make what is already the most important attribute for wizards even more important.