The main thing this would do is make some spells, like Charm Person, maintain their usefulness over the life a spellcaster's career. Which is, I think, the reason that they didn't so a system like this. They were going on the idea that more powerful spells should be harder to resist rather than the idea that more powerful casters should be harder to resist.
So it really comes down to this:
Do you want your 15th level fighter to be taken down by a charm person cast by a 16th level wizard (DC 26 = +8 {Int 26 [Headband of Intellect +4]} +8 {CL/2}). Even with a high wisdom, a fighter's Will save makes this a very dangerous spell.
Not saying this is bad...just thinking out loud.
DC