Are they mandatory? Or can Alteration help with that?

The master level alteration spell Dragonhide gives 80% damage reduction, the very same maximum that armor would give. So in a way, armor is not needed.

Unless you're a mage/archer. If you plan on fighting in melee combat, you will get shredded with no armor. Alteration does help with or without armor.

Ebonyflesh with all 3 mage armor perks gives you an armor rating of 300, which is quite decent. However, the mage armor perks only work with clothes.

Dragonhide maxes out your armor rating irregardless of what you're wearing or which mage armor perks you have. However, it is useless without the Stability perk. It comes in handy if you are a warrior who wants to zerg rush those damn OP Draugr Death-Overlord ebony archers who shoot .50 BMGs from their bows.

If you're a high-powered illusionist with the backstab perks, you can get by OK without it, I think. Just stay invisible and muffled, then one-shot everything you want to kill, and hit everything else with a good Fear spell.

But if you're playing DiD, I recommend armor, kind of like a net for your traqeze artist. Accidents do happen.

... zerg?

Even if you're a warrior wearing only clothes, you don't need armor per se, but you do need a way to mitigate damage. You can do this with blocking as well if your skill is high enough and you've invested the right perks. You also have to be good at blocking as a player.

It depends on your playstyle. As a warrior, its pretty much a necessity.

http://tvtropes.org/pmwiki/pmwiki.php/Main/ZergRush