If armor is tied to spell efficiency, as we've seen in the past, that's the advantage.
I really disliked this system. The idea that armor somehow had a negative effect on magick. I understand the reasoning for balance, but it's a poor choice.
The reasons mages don't benefit from heavy armor is that 1) it's a skill that takes away from training in other areas 2) when it does level, it increases your Endurance, which has little to do with your ability to make spells. Obviously we don't really know how the whole Attribute system affects leveling anymore, so let's move on.
Robes have often been worn, in history, as a sign of priestly or knowledgeable people. So for one it's an aesthetic thing. You could give robes an enchant bonus that is equal to say, all the pieces of a suit of mundane armor combined, so that they do not miss out on those slots.
Futhermore, the idea that a mage would wear heavy armor for protection is somewhat ridiculous. His fatigue and health are generally lower than a fighter, and heavy armor would simply be a turtle shell. Mages benefit from being far away from their opponents, or preventing their opponents from harming them with magickal means, either paralysis or shield spells.
Of course, you do get hybrid classes of battle mages and so forth who do combine heavy armor with magic. They will just not have as much magic power as a pure mage in a robe that has the same number of enchant points as a suit of armor.