In practice, if you are playing a mage "properly" (if there is such a thing), then you will not be getting hit... thus, you will not be gaining skill in heavy armor, and that stat becomes stagnant/irrelevant after a while. This becomes even more apparent if you are able to skillfully place summons tactically or use stealth/illusion to disrupt crowds of mobs and control the flow of combat. Try it out... use destruction magic as well as the support system (conjuration or illusion) as the game "intended", and see if you don't stop getting hit.
PS To avoid your mage getting boring, don't use fortify school enchants on your gear to reduce casting cost to zero. Having a finite supply of power will make even a very strong mage still challenging and fun to play.
I disagree with a lot here. For one, while you can avoid getting hit and in fact not getting hit and thus not gaining armor skill is an issue, armor is still 100% superior to robes + alteration. Spending more mana on something you can get passively is just not a good idea. Also, by default, a set of any armor gives +25 armor per piece regardless of skill. That's the equivalent of ebonyflesh as a freebee for just putting armor on, on top of the armor's shown rating. Then there's the question of mobility, light armor has almost no noticeable detriment. If you're using illusion muffle completely negates its penalty to sneak as well. A set of armor only has to give 200 shown rating to match 3/3 mage armor and the strongest armor spell.
You could argue that with alteration you'll eventually get Dragonhide, but then until you hit 100 alteration you're stuck with the basically worthless unbuffed armor spells. You won't even compete with unimproved fur armor until 75 skill, and you're still losing that competition at 75.
There's also the problem that when you have the time and mana to cast dragonhide, you're at a point when you don't need it. I'd only use it if I were intentionally going into melee range, but if you're aware of an enemy you generally don't have to do that to kill it as a mage. When you really need armor is when you're not prepared and/or at the early stages of the game when your health is low. That base 100 armor rating from any light set is more vital to any DiD character than Dragonhide ever will be.
I also don't use cast reduction enchants, and honestly it really isn't challenging or fun at high level. After hitting a certain level of illusion and conjuration(50, but 75 is when it
really gets overpowered) everything is easy. There's a certain novelty to watching dremora lords cleave through everything but it's not particularly exciting. I shoot things with a bound bow still but at this point it's unnecessary.
Another thing worthy of note is that due to (destruction)mages not getting more magicka by default, you have an innate disadvantage if you use the same magicka pool for damage as you do control, utility, and healing and so on. You're better off using a bow, really. And with summons, you can have permanent and/or longer lasting ones that partially negate the cost since you can have them out most of the time before combat while your magicka regens several times faster, while high level destruction just gets more and more expensive.