I love the idea of robes. I think they look really poorly done and a little tacky when found in most video games, but it is to the point that I draw pen/paper RPG Mage & non-mage characters with some fort of dramatic flowing madness. Like heavy armor but adorned with vestments, over-sized cloak, maybe even a decorative loin-cloth Warhammer 40k style. I don't find robes and such to be "dresses/skirts/etc" the idea of pants is pretty new for society, and an idea that I think is highly overrated.
That being said, the game designers opened up a can o' worms by giving the lame excuses for why mages can't wear armor, like the two given below.
Like the metal restricts casting, so why are so many magical trinkets made of metal? Shouldn't all that arcane jewelry just be made of non-metal products, and then could they wear wooden armor like 3rd ed Druid? I know its based on the European mystic traditions (ie rosicrucian/golden dawn/etc) that stated that iron is like a magical energy sink, like horseshoes absorbing bad luck. Gold/Silver are non-obtrusive and could in theory be used to further focus/shape the energy. But simplifying it to they can't wear metal has issues.
Armor restricts movement. This one has less holes, but unless the designer is going the Hindu type route that somatic components are full on yoga moves (which I've never seen it depicted as that in any game, though it'd be kinda cool), I don't see how those somatic gestures would require more freedom of movement than archery or mobile melee combat. Think about the positioning and range of motion required to make a proper pull on a bow, I think that'd be hard to do in a suit jacket let alone armor. It was obviously done through history, so good on those guys, and why can't the mage flail his arms and do a simple jig in chain?
As game balancing it totally makes sense, they could look awesome, but most of the time are just phoned in. Compare any of the DragonAge PC robes aesthetically to Morrigan's robes...bastards I want cool robes damn-it!