It can be worth training Armor, but for my casters, that usually comes really late.
Armor rewards you with “free” AC–i.e., as you skill up, it gives you a couple AC points based on your gears’ combined base AC. So for example, Cloak is 1 base AC, Steam Dragon is 5 base AC, etc. The enchantment level isn’t considered for this calculation. So +5 Steam Dragon and +0 Stream Dragon benefit the same amount from AC skill.
Add up your combined AC and it’ll give you a sense of how much of a benefit you get from Skilling up. If your base AC is low (say, you are wearing Robes), the bonus is also going to be low. You can figure out how much Armor Skill you need to get another point of AC offline with WizMode, or online with !a, I think (I don’t play online much, so someone will probably come around and correct that if it’s the wrong command). 5.5 and 11 are often good breakpoints for casters (5.5 gives +25% to your base, 11 give +50%). So if you have Steam (5), Cloak (1), Helm (1), Gloves (1), 5.5 Armor gives you +2 pts of AC (25% of 8 is 2). If you have just Robes, Helm, Gloves, 5.5 gives you only +1 AC. Anything higher that 11 on a blaster probably not worth it for a 3 rune caster. The xp will probably do more good going into guns or HP or Evoc or Invoc.
Armor also reduces encumbrance, which penalize a bunch of things, but I don’t bother doing that math for my casters. I just put it on and check the % and defensive drops. As a general rule, I aim for Troll Leather or Steam Dragon for my casters. I might wear Ring Mail or Acid/Swamp/Fire/Ice Dragon depending on the build, gear, artefact benefits, etc., but that is up to the generosity of the RNGs.
So Armor gives better return when you are running around in heavier armor. My blasters are usually not ready to wear heavier armor (e.g., Fire Dragon) until much, much later in the game (like, Vault or Depth time). Armor takes a backseat after main guns, HP, Evoc/Invoc, Dodge, Shield.
Lastly, I definitely invest in EV for my casters, even with the ways around it. Ideally, I want at least ~10ish for everything by the time I go for a rune (more is better, ofc). Btw, there are ways to get around SH too–e.g., beams.
Even “small” layers are good. Let’s say your AC is enough to stop 25% of the hits, you EV also stops 25%, and SH is the same 25% chance. With one layer, your enemy gets through to you 75% of the time. With two layers (say, EV + AC), they get through ~55%. With three layers, they are down to ~45%. That’s why I stack as many types of defense as I can. Esp since the first few levels of a Skill are pretty cheap, even with cruddy apts.