Bah, it doesn't advance spells known or spell slots, so it doesn't even matter 99% of the time. It's like a free Practiced Spellcaster for multiclassed casters. Who really cares, other than a few outliers like maybe Nar Demonbinders, Sublime Chords, Ur-Priests, etc?
I think it would probably be better if you actually let it advance spells known and spell slots, and counted all classes outside of your casting class as +1/2 CL (just exactly like how initiator levels work). This would be a big help to people who want a few spells, but don't want to be a full caster.