At the moment casters rarely multiclass for the simple reason caster levels make up for spell damage. losing caster levels means a significant drop in damage done.
just like taking caster levels doesn't mean you don't get any increase to BaB, but rather get a smaller increase to it, I believe that taking non-caster levels on a caster shouldn't give you no increase to your caster level, but rather a smaller increase. (I believe BaB increase is two thirds for caster levels, so something along those lines)
It has been suggested before me, but I could not find the thread so forgive me for starting a new one.