As stated in the subject, Arcane Defense is a feat that I doubt anyone would actually take, furthermore, most of the time GSF is not applied to every spell used, making a specialist spellcaster in certain school just as good as any spellcaster, for many spells. Giving those who have GSF arcane defense feat makes them automatically more 'specialized' in that area, making a lot more sense and rewarding a bit more those who have spent 2 feat slots on a general basis for their field of expertise. :)