I do like this concept. This would certianly control the level of power a caster level would have. Access to spells would have to be worked into this, otherwise you're just a sorceror.
I'm in agreement with those who don't think the spells should be more powerful and such. Basically I think multiclassing should be for specific archtype purposes. A fighter with counterspell, a rogue/thug with a greatsword, a cleric who dabbles as a warlock.
Most of the issue would be fixed...