For farstrike (earlier version) I think the way the AI thinks is "Out of my tests I kill the 10 protection infantry 80% of the time, but I kill the 0 protection mage 100% of the time, on average dealing more hitpoints of damage if I target the mage". Shamblers are some of my favorite mage decoys for this reasons, because of their hitpoints, protection & MR they'll often be targeted by mages. Of course it's prohibitively expensive to use them as, say blade wind decoys.
