I think a better question might be; is the any point to wearing robes?
But the EVEN BETTER question is... does a pure mage even have to wear armor? The answer is a resounding NO!!! If you've advanced in multiple schools of magic to at least the expert level and smartly distributed your perks, a pure mage character becomes untouchable in battle. I ever HARDLY cast ebonyflesh anymore because I can take multiple enemies out at a distance with destruction, illusion, and conjuration with expert level spells at a distance... even in dungeons. If you have to wear armor and you're a pure mage, not a hybrid like a battlemage, then you're not very good IMHO.
A robes faster regen is very useful when your character has a large mana pool. Mine has a mana pool of ~700 with augmentation. If my regen is augmented at 1.7 (~70% regen from enchanted armor from max enchanting+ base in combat), than my regen rate would be (0.017 * 700 = 12). If my regen is 2.5 (150% with Master robes + base value) my regen rate would be (0.025 * 700 = 18) These values are assuming rounding. After 5 secs of battle that's 60 with enchanted armor and 90 with robes. It would take an extra ~2.5 secs for the enchanted armor to catch up. The ability to cast higher level spells quicker or cast lower level spells an extra time can mean the difference of winning or losing many battles in the last second.
Now, I realize that if you set two school to zero via enchanting, that is a moot point, but IMO, that makes the game play pretty cheesy. This especially goes if you set destruction to zero and spam impact over, and over, and over again. IMO, they should have kept cost reduction in no more than two slots.
An armored "pure' mage... ambiguous term I know...is probably one of the most OP characters you can make. You'd be whittling down the vast majority of enemies before they even touch you and once they do, they take a sliver of health. If you want a god-like character, then go this way, but I can't think of a more boring playstyle. Maybe a tank melee character that dual wields 300+ dmg to opponents or a sneak characer with 300+ archery ON TOP of sneak bonuses. The armored mage definitely falls in this category.