Bad idea. Mages will die much quicker if they only get a piddly bit of DR, since their class is not likely to get much of a defense bonus, having no significant combat training.
Mages already have access to spells that grant DR, like Stoneskin. Why is it that more of their defensive spells don't give DR? Because the most important thing about a mage's defenses is that he or she DOES NOT WANT TO GET HIT IN THE FIRST PLACE. When the orc berserker gets past your stupid meat shield because he's too busy dealing with other members of the orcish horde, your mage is dead if that orc hits you, whether or not you have a stupid little worthless DR 2 or DR 4. Your piddly d4 HPs aren't going to give you a chance against that orc barbarian's raging greataxe power attack.
Your Mage Armor, Shield, Alter Self, Cat's Grace, Blur, Displacement, Haste, Gaseous Form, Blink, and other spells all serve to KEEP YOU FROM GETTING HIT IN THE FIRST PLACE. Because you are made of glass.
SO, don't change those spells. They can use Stoneskin or polymorphing if they want DR. But the other stuff needs to remain an AC/Defense bonus that makes the mage harder to hit. And don't change Bracers of Armor either.
If you just want to avoid some kind of Mage Armor abuse with the armor-as-DR rule, then just say that Mage Armor does not stack with actual armor and other armor effects, so only the higher armor bonus applies as normal. So someone wearing leather armor and under the effects of Mage Armor would just get the +4 AC/Defense from Mage Armor, not the DR of their leather armor. Treat Bracers of Armor the same in this regard; they're based on Mage Armor in the first place, so they work like it.