Wyrd Greyhawk. Armored wizard, no. Well, maybe a little.

When I make decisions about how I'm going to house rule The Game, it primarily setting specific. And it's not usually about balance in the rule set itself, it's about simulating in as realistic a manner as I can, the conditions of life in the game world.
I'm still not clear on the accepted definitions of simulationist, gamist, narativist, etc... It seems that everyone operates from their own definitions here, and the whole things seems to vague for me to argue about.
I really need firm footing for me to bother with discussion, and until somebody publishes a Websters for Gaming, all I feel confident in doing is expressing opinion, and explaining why I hold it as best I can.
Most of the arguments I read online boil down to differences in definition, as often as they do differences in philosophies.

So, Armored Wizard.

Not for Wyrd Greyhawk, with few exceptions.
Of course, there is the game balance consideration that you don't want a character class with access to powerful magic dominating the game and over shadowing all the other classes. Armoring a wizard turns him into a magic tank and if you want your players to work together, it's best not to let any one of them have too much power.

My view of magic use, at least in Wyrd Greyhawk, is that it's a skill and an art that is developed slowly over time and requires enormous devotion and concentration to master.
Magic use is not easy, and very few can learn to do it at all. A magic-user, of whatever stripe, can not afford to spend the time to learn many other skills if he ever wants to advance in magic use.

In addition to the fact that large amounts of iron in close proximity affect the ease of magic use, essentially acting as a grounding connection for the arcane currents the magic user must draw into himself during casting, wearing and moving in heavy metal armor is as much of a skill as wielding a sword, and this is a skill a magic-user has no time to master.
Metal armor drags on the limbs, it requires a great deal of physical energy to move in for any length of time. It requires time and effort to get into and out of, and to maintain. It costs a great deal of coin to purchase and repair.
These are all costs the magic-user can't afford to bear if he intends to increase his skill at magic use.
A wizard in Wyrd Greyhawk isn't going to wear chainmail any more than a surgeon would in the operating room, or a lawyer would in court. It's not a tool of his trade, and would really only get in the way.
You could wear a suit of armor while mowing the lawn, or fixing a computer, but it would make your job a lot more difficult and if it's your job to fix computers, you don't need to wear a lot of steel to turn sword blows. If a magic-user finds himself in a position where wearing armor is the only way he can survive, he's a crappy magic-user and is advertising his lack of magical talent.

That being said, I have some exceptions.

Specialist magic-users, like the Alchaemist and the Viviomancer, I allow to wear soft leather, (AC 9,AC 8 with studs), leather equal to a motorcycle jacket in protective quality. I let specialists do this on the assumption that their tight focus on a small area of magic doesn't require of them the all consuming concentration more encompassing spell work does.

The second exception is for NPC magic-users of Imperial Suel blood lines. The aristocracy of the ancient Suel Empire was steeped in sorcery for thousands of years. They deliberately engaged in an incestuous program of arcane eugenics for centuries in order to increase the power of their wizardry. They trained in martial arts alongside the eldritch. The result of this is that humans of Imperial Suel blood function as fighter/magic-users without any penalty.
This branch of the Suel race isn't open to PCs though. I use them solely as antagonists and NPCs. They are extremely few in number now, and most of them are quite mad, as well as evil beyond redemption.

I do let magic-users carry and use swords, but there is a catch here too.
Again, swordsmanship is a skill the magic-user will not have time or energy to spare to learn, so, a magic-user can only employ a sword in most basic manner.
As far as the rules go, this means that no matter the magic-user's level, he always attacks with the sword as a first level magic-user. No advancement in skill is possible.
This doesn't apply to traditional magic-user weapons like the dagger or staff. The magic-user's chance to hit with them improves with level advancement as normal.
I'm operating from the assumption that the traditionally allowed weapons of the magic-user class are actually used as aids in learning magic when the magic-user is a young apprentice.
There's nothing written in the books that says so, but I find it a convenient rationalization.

And that's good enough for me. Heh...

Now this only applies to my version of Greyhawk, mind you. In other settings I could easily change my rationalizations to make the facts of magic use fit the game world.

I would have no problems with armored wizards in a more magic-rich environment. Wyrd Greyhawk is low magic, earthy and desperate. On Carcosa, or even maybe the Forgotten Realms, I'd be more free-wheeling, maybe.
If the classes available to players were restricted to just magic-user,and fighter, or if the world didn't have distinct classes and each PC was built from the ground up, I'd be more open to the armored magic-user.
But then again, that's moving far afield of AD&D, and I wouldn't stay there too long.
I try other systems, but I always come back to my foundation, I am Old Guard after all.