In the context of Mystara and the Thyatian / Karameikos lands, there's one possibility I've thought of that doesn't preclude the existence of other magical systems: reliability.
Thyatia is characterized as a conquering empire, one that looks at the utility of all things -- including magic. It's arguable therefore that the Vancian magic system, with:
- verbal, somatic, and material components;
- spell books;
- memory space requirements;
You'll know if you've successfully memorized something or not. You'll always cast it successfully (assuming all components are doable) if uninterrupted in combat. You'll always know how many spells you've got with you and how many more you can cast that day!