You've got to feel sorry for the average orc. We're coming at them with tons of different skills and in groups mix and matching all sorts of different ways to slaughter them. They have auto attack and maybe a debuff or two. Sometimes they have a heal.
Imagine if there were enemies in balanced groups. The healers were really good at healing but really vulnerable themselves. Archers that did lots of damage and kept their distance, while letting the tougher melee troops hammer away. The basic behaviors are all already in game, we just only see them in special circumstances.
I think the reason we don't see enemies fighting like this out on the landscape, or at least in their camps, is a balance one. If enemy healers are much better at healing than they are now, but also much more vulnerable to burglars stabbing them in the back, obviously Burglars are going to only want to fight them. If tanky enemies are balanced to counter burglars, they're going to be annoyed when they have to fight one of those. If enemies have counter-classes, the smart thing for everyone to do would be to just skip the annoying ones that counter them and cherry pick the enemies they can quickly destroy.
Group up classes of enemies like boggarts or warbands though, where we only get rewarded when the main enemy is defeated, and there can be incentives to have a proper battle.