Thanks for the comments Afrodyte. I did modify the normal D&D spellcasting system and classes pretty extensively, I didn't include the modified classes in this document. Basically, there are different cultural types of magic, so the wizard and sorcerer from the PHB don't exist. Instead, I made up new traditions of magic that use either arcane, spiritual, or infernal power- each with their own spell list, special abilities, and limitations. The new classes include the alchemist, cabalist, classicalist, diabolist, Romari medium, runecaster, sorcerer, and witch.
To answer your question, I hadn't really thought of making the casting roll a level check. I suppose it wouldn't really make much difference to the end result, its just that I usually equate a level check with the raw power of the caster (like for overcoming SR), while the skill check represents the caster's knowledge of his area. And since the skill check is based in part on level (with max ranks equal to level +3), I chose to go that route. I also tend to use the Knowledge: Magical Tradition skill a decent amount during play (it replaces Spellcraft among other things), so I thought it might be a good way to increase the applicability of the skill. Surprisingly, even with the greater flexibility offered by spontaneous casting, most of the spellcasters in my game memorize most of their spell slots, leaving only 1/4 to 1/3 open for spontaneous casting.
