To be fair, only this part is “possible” 
Sure, but I guess my point kind of stands, that the enemy isn’t opting to do this on purpose - it just has one objective, which is to rush you, and if it can’t do it, it doesn’t move. That’s not the same as actively defending.
Hey ho, we can dream.
Exactly. First time when I saw this in a mission, it kind of killed the immersion. It just becomes another meta tool in how to cheese AI into doing what you want.
Ability for AI to change tactical approach, from an assault to a defense from a defense to a counter attack would greatly improve overall impression and longevity of the game. This could work well with the additional spawn coming from the map edge to repel your attack, or having few last enemies “barricaded” in some spot, forcing you to assault them before reinforcement arrives. Sadly we probably won’t see this.
Agreed. Even if this was arbitrarily cheese-baked into the logic somehow, i.e sometimes they attack, sometimes they defend, for some unknown reason, it would change things up a bit.
It may be the case that just like the human player, often, each AI unit is pinned (apart from melee suicide types), so we never (or rarely) get to see the fruits of their inner machinations. It seems as though each unit is its own agent though, and that they aren’t working cohesively, other than obeying the directive to not bunch up, which sometimes would be interesting to see as a law broken.