I agree, too easily exploitable, at the very least i'd say only apply it to a single casting class.
I don't see it as two sorcerer levels = fighter level.
The sorcerer while primarily dealing in magic, does do some combat such as rays/touches or if you are low enough level, crossbow.
A fighter-- the most he ever really uses is his +2 orc bane longsword, so it makes sense not to improve spellcasting ability