I discovered something today after a person playing Sedna on my team left the game.
THE AI IS AFFLICTED BY THE ABILITY BUG.
When I was in combat fighting another hero AI controlled Sedna was nearby. She had already healed me once already (then returned to attacking nearby creeps instead of helping attack the enemy hero but whatever, at least she healed me). I saw her turn again towards me, presumably to heal me. And she froze. Yes, she had been stuck by the ability bug!
I fled the area as I was losing the hero battle and noticed Sedna continued to stand there, transfixed in the wake of the bug. The enemies proceeded to attack and kill her, without her putting up any resistance whatsoever. Because the AI, unlike a human who knows when the ability bug strikes you need to move or you'll never do anything, has no idea wtf is going on. So they do nothing (or are continuously attempting to use an attack ability like pounce). And die.
I got to thinking - this explains a lot. If the AI has been encountering the ability bug since day 1 perhaps this explains their general ineptitude and also why certain AI heroes are seemingly incompetent to the extreme. I hope that the eventual patch that fixes the ability bug will also translate over to the AIs, making them just a bit better.
I still think the AI shouldn't even exist in multiplayer, but what can you do.