If mages had thier own specific armor skill I would expect it, while at 100 skill level, to eaither provide the same protection as a Warriors armor skill at 100, or provide a equal yet slightly diffrent armor rating, such as less physical damage resistance but more elemental resistance.
TES games have never had that level of symmetry. In Oblivion, the only way to unlock a chest that was underwater was with a pick, so lockpicking had an advantage over unlock spells. It was also less detectable. In Skyrim, no matter how much you perk your alteration you are not going to get your mage armor to be as good as a warrior with well smithed armor and perks in the relevant armor skill. Max you can get is 300 (more with the Master alteration spell but it has a very short duration and very long casting time so again is not as good as a warrior with a full set of well smithed armor).
I am not sure that level of symmetry would be a good thing anyway. The fact that some classes are better at some things but not as good at others is what makes the game fun.




