LSP, this is not at all in response to your post, I just got done writing this out is all.
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There are a lot of good reasons to engage the CNS initially.
It can bypass armor or an explosive vest, if either are believed present.
It can raise a shot trajectory to mitigate danger to bystanders, especially in combination with a lowered shooting position, even as simple as a deep crouch.
It may stop the threat with fewer rounds fired into the environment, mitigating danger to bystanders.
5+ seconds may be required for a determined threat to stop from loss of blood pressure resulting from high thoracic cavity shots. The involved human dynamics all happen far inside that 5+ second window. 1.5 seconds = 7 yards of distance traveled, an untrained person can fire about one shot per quarter second, and if the hand is already on the gun, almost no matter where the gun is - in waistband or holster, held under arm or down by leg, even pointed in the complete opposite direction - a gun can be pointed at whatever other direction or thing and fire the first shot in about half to three quarters of a second.
There are a great many situations where we quite predictably might not have 5 seconds to wait for that blood pressure loss and subsequent unconsciousness. A CNS hit is the closest thing to a guarantee of immediate cessation of deadly threat, IF we have the skill to make the hit under the circumstances at hand.
But a failure drill pattern of engagement also has a lot of merit. The initial high thoracic cavity shots afford a potentially faster first shot, while the margin of error (C-zone) mitigates imperfect shots that would be misses relative to the head, and those initial body shots might facilitate the ease of a subsequent CNS shot if the threat has not stopped yet. In addition, if the threat is in fact seriously psychologically affected by being shot or shot at, which while unreliable, does happen in some instances, then we get that ball rolling ASAP. In its own way this plays into an important role that speed can end up playing.
I think both body-then-head and straight-to-the-head are very responsible and effective engagement sequences.