Not to mention if you wish to take and use the Mage Armor perk you can't wear anything with an armor rating. Just depends on playstyle.

who says they're playing as a pure mage?

Agree. 44 expert with almost 600 magicka and I′m doing fine as long as I don′t get touched, wich is rather easy with the perk that stun the enemy.

I have pure magic restoration outfits

there is no penalty to your casting from wearing armor.
the penalty is simply base speed, stamina consumption, noise, both light and heavy armor have perks to remove those conds aswell.
having that said some things to consider:
-there are perks in alteration tree that make cloth armor just about the same as light armor (aslong as you only wear cloth), that only cost 3 perks, investing in any armor will cost you way more perks than that, so if you are planning on using alteration for defence, its worth having this into account.
-it will be much harder to kite/train mobs in heavy armor for the majority of the game (perk is further in the perk tree and takes a long time to get and many perks sacrificed to get it)
-dont think that heavy armor = capable of withstanding melee attackers, on master diff. without block/shield and melee weapons to stagger and stuff, you will still die fast, you just wont get 2-3 shoted anymore, for a mage with armor and alteration, paralysis is still the best defence, not its armor.

armor does help, simply because mage gameplay is more demanding and challenging than any other, and mistakes are made, armor helps those mistakes not being so punishing when you take hits. Im leveling heavy armor myself.
dont see the point of light armor... its a melee/bow dps armor really for the stamina regen, heavy armor gives you much better protection and an anti stagger perk.... much better.

are you wareing any Armour or running round NAKED (robes = Naked)

its not the arrow hitting just above the target thats aleays been there its the is an enemy is X disrance from You your arrow goes straight through the NPC alerting the room to your presence also theres no headshot damage multipler

You mean a Sorcerer. Not all Battlemages wear armor.

I'm tanking pretty darn well on Master (yes, my character's name on master difficulty is Master) using heavy armor. I mostly used block until I got the ebony mail with the other pieces, but after that I have been using a shield less and less.

As a melee character learning about this 0 cost casting, it looks good to approach it as a battlemage. I have high health and stamina, very high defence, and now I can just switch to magic with almost no penalty.

just robes, no armour, still doing great (but i have found one thing that can kill me, the falmer bug things that always seem to kill you no matter what type of character you play)

i heard about the glitch after i posted that, still have not suffered from it, and bows are my main weapon