The AI is using standard units - my army just fought an army from Altar that included Defenders, which have leather armor.

There seems to be a good range of units available already and I think this area will improve as more units are added - as Tasunke was saying, the zombies approach is pretty effective at the moment and the AI should also have access to low cost zero maintenance units to bulk out its armies and garrisons - the units can be produced very quickly. Eventually they get superseded but you can get loads of mileage out of them

I think the AI should be trained to do it  (i.e spawn 2 units of chumps per town a turn) or they should nerf it via minimum maintenance costs or whatever.

With regard to armour I don't think it's always a good idea to build units with it - ranged units don't need armour as long as you have other troops. Rubbishy conscripted peasants dont need armour either. Shock troops do need the best equipment available though.

archers need armor if facing enemy archers

@ Tasunke

Sticking armour on archers increases build time and maintenance costs alot so I think normally its best to keep em cheap - I haven't checked the exact costs but I wouldnt be surprised if full leather armour doubles the labour cost and maintenance of an archer unit. I guess if the AI is spamming archers it might make sense to stick armour on your archers

My enemies are very fond of Spear Maidens, which is a solid design (I always use Battle Maidens with same gear as Spear Maidens but more offensive with Bloodthirsty, Charge and Fast).

In my last game it lasted enough to actually see my enemies using some warg cavalry with some metal armor.

 

Pioneers should not appear in armies. I can abuse enemy pioneers to heal my army to max in one turn using Heal and/or Regeneration as they do not attack. Plus it does not make sense (I cannot have tactical against caravans so why Pioneers even if they are part of an army?).

Good points. Maybe weak units shouldn't appear in combat, to help balance their lack of upkeep.

I always give my archers leather armor, as long as there is no initiative penalty its only a good thing. As long as you have high enough production anyway.

Of course in my games I was fighting waves of dross archers.

Archers need numbers and initiative, not armour. The only real armour I could use for my archers is that robe with +2 initiative. (in the last game, just because, I upgraded some to heavy armour but at that point they could have gone naked and it would have had 0 impact) That robe and some armoured meatshields to block beasts of movement 7 and similar.

Wintersong is right about the cost drawbacks, but I usually equip them with leather in the mid game just out of paranoia. It is a good habit for when they increase the AI abilities.

I use a few armies of elite archers in the late game. In the late game you need armor on archers in addition to Initiative. I use chain arms, head, boots, monk robe, and leather greaves; Yew Longbow, and Wargs if I have extra. Armor is necessary to avoid losing any units from the stray army of only archers. By that point, the late game, money doesn't restricted unit quality. Resources are the big limiting factor.