I've played Skyrim for 3000+ hours now, and this question has preyed on my mind much of that time. Given that Tamriel is a hack-and-slash world (well, Mundus actually) where Magic works, there seems to be no penalty at all for a mage to cast Magic AND wear armor at the same time. So why, why, why, why would anyone willingly risk his neck and other bodily parts to the ravages bestowed by hackers and bashers by NOT wearing armor? Nearly all robe-type garbs have a ZERO armor rating. Worse, most magically imbued robes are specialized, restricting the robe's magical properties to just one School of Magic.

I could see the eschewing of Light and Heavy Armors if wearing those items diminished a spellcaster's proficiency. But from what I have seen after thousands of hours of gameplay, there is not one iota of ability lost by wearing armor while casting magic. Shouldn't simple self-preservation motivate nearly all magic-users to improve their survivability by donning at least Light Armor? If anything, encountering a spellcaster wearing just robes should make most warriors go weak in the knees just thinking about the fact that the wizard is soooooo powerful, he has no NEED to wear armor. Instead, I find that when I encounter robed spellcasters, I'm thinking "Goody! Easy pickings!" And I imagine that pretty much all hack-and-slashers would be thinking the exact same thing. So the hordes of spellcasters wearing robes instead of armor must be really, really stupid to think that they are gaining any benefit by wearing robes anywhere except at the College of Winterhold.

Additionally, since nearly all apparel can be imbued with one enchantments, the robe-wearer is cheating himself out of the ability to be wearing four or five (body, head, hand, feet, and shield) rather than just two (head and body). [EVERYONE has the ability to also wear a ring and a necklace at the same time.]

To motivate a spellcaster to wear ONLY robes, there really has to be something in effect that makes it more beneficial to willingly sacrifice the body protection provided by armor. Something like, "Thou shalt not be able to cast spells while wearing armor," or "An armored individual will NOT be able to employ a magical staff while so armored." Something.

“No matter how subtle the wizard, a knife between the shoulder blades will seriously cramp his style.”
― http://www.goodreads.com/author/show/27704.Steven_Brust as Vlad Taltos in Taltos

I often find myself asking the same question.

Honestly, it made much more sense in previous games where spells either weren't as effective while wearing armor or mages were barred from using certain materials/armor styles. I don't believe Morrowind had that limitation, though, so it's hard to say exactly what the hell is going on.

(my thoughts are a bit scattered right now so bear with me)

My guess is they can't spare the time for training to wear armors that would most likely hinder spellcasting. One would imagine that, realistically, it would take a long time to learn how to use heavy armor properly (the Sorcerer Class from previous games supports this). It's a bit of gameplay and story segregation. Sorta how we can crank out a full set of plate armor in one second when it should take weeks or even months for certain materials (and some requiring very specific conditions and yet in Skyrim we can just craft them in seconds if you have the perk). In Skyrim especially, we can pretty much start using any skill without problem. Armor spells/enchantments could theoretically make up for lack of physical armor and therefore much more convenient since you don't have to lug around sheets of metal with you. But, again, Morrowind screws everything up since not only does armor not give penalties, but the system actually encourages learning an armor skill since it could hold more powerful enchantments.

Honestly, I think Oblivion had it right. You got casting penalties for wearing any kind of armor and it took a long time to even be able to cast spells at full power while wearing armor. At least it made some sense.

Ego...think Neloth. Being able to shoot fire from your hands and summon creatures to do your bidding messes with a mages mind to where they feel so powerful that simple swords can't scratch them..until a sword wielding warrior severs their crazy head from their shoulders

From a gameplay standpoint I think removing the casting penalties was a poor decision, for all the reasons you and monkeyemoness mention. Gameplay-wise I agree that there really is no good reason to wear a robe in Skyrim.

But from a roleplaying standpoint I've never understood why wearing armor would impede one's ability to cast a spell. Does leather and metal interfere with the mage's ability to concentrate or focus magical energies? I don't see why. Roleplaying-wise, armor penalties never made sense to me.

So I'm of two minds. On the one hand I understand why some people would want armor penalties back. And I wouldn't complain if they did. But on the other hand if armor penalties do come back, it will still not make any sense to me in roleplaying terms.

I wanna say it's because of how Morrowind deals with it (metal absorbs magicka--some more than others--so it interferes with somatic spell components) plus probably exhaustion/encumbrance from the armor(again, failure rate in Morrowind). Maybe it's just like trying to perform basic motions and activities while wearing a ski mask and gloves. It's not as easy as with your bare hands and head, but you could get used to it.

But this is all just speculation because we never get to know how magic even works in-universe.

Magic, don't question it.

I run ACE-combat skills, which restores casting penalties (% chance for more magicka to cast a spell and slower regen without perks) so it makes sense not to wear as much armor as a mage (pure mage, anyways).

I suspect that part of the reason is that, gameplay-wise, you just can't carry very much. You have to put points into Magicka to cast spells even with cost reduction via perks, and if you don't put points into HP, an archer can one-hit kill you no matter your armor rating. Which means stamina - and therefore carry weight - is neglected. So you have to do things like casting a -Flesh spell to bump it up and compensate.

Canon-wise...nooooo idea.

The regeneration provided by robes is much higher than can be enchanted onto armor (barring the Vampire Royal Armor and abusing alchemy); as well, magic skills require such a high perk investment for use that, while a mage could wear armor, they wouldn't be very good at it.

Insofar as Mage Armor goes, three perks there versus five or greater for another armor skill (though for a reduced armor rating), and Dragonhide grants the maximum armor rating without any armor skill.

As well, wearing armor slows movement.

I can see the pros and cons for user penalties and how implementation would affect gameplay. To be honest, I have a hard time imagining any player ever actually wanting to play as a pure mage (no armor). Or rather IF a player did try his/her hand at being a pure mage, that they would find that to be much, much, MUCH harder to stay alive than pretty much _any_ other skill focus (Rogue, Fighter). [As far as I can see, Cleric and Mage function identically and are for all practical considerations, just the one Mage class.] But that leaves the hundreds of NPC mages running around in their robes. What were they thinking? Even allowing for ego-warping making them feel invincible, when starting to study Magic, the NPC _must_ have seen all of the "real world" evidence that shows just how overwhelmingly the balance tips in favor of armored characters when it comes to survivability. Sure, the occasional high-level wizard may carpet a battlefield with dead armored foes, but for every one of those, there's a couple thousand developing Mages that have been hacked down by armored thugs. [Or more likely, armored mages like the Thalmor.]

Design-wise, I am inclined to believe that if Bethesda had kept meaningful armor penalties -- % spell failure, longer casting times, diminished potency, etc. -- then hardly anyone would bother to even use Magic at all. In which case all of the development resources expended on creating the Magic elements in the game would have been wasted. And with less Magic in the game world, the game becomes less Fantasy and merely Dark Ages-esque.

Strictly speaking, mages have a good deal of longevity with proper perks because healing spells are quite cheap and have a high magnitude, not to mention summons providing a distraction. I played a mage that just used Mage Armor and it wasn't much harder than anything else. In fact I might've considered it easier due to having a lot more versatility, and certain tricks, such as preventing dragons from shouting by draining their Magicka with lightning spells.

Enemy mages have deliberately fudged stats, which give the higher-leveled ones all three ranks of Mage Armor, Ebonyflesh, and high health, to boot.

I've always thought that the metal of armors disrupt with channeling of magical energies, thus making spells more expensive to cast, results them in being less effective, or they may fail outright. Or something, i know more about science than magic

And I believe armor confers a magicka regeneration malus in Skyrim?

Personally i always play a battlemage with light armor (if possible). In TES reinforced with Alteration, or in Skyrim, enchantments.

I always liked my battlemages with a heavy armored chest piece and a robe, course I can't do that in Skyrim.