Squirrelfighter, one crucial point of your argument makes no sense... theoretically, if you locked your muscles so your head did not spin at all and you were completely rigid, then your brain would be completely protected. In this case the force of the strike would cause your flesh and bones to take the impact.
In reality, the reflex is to roll with a connected punch, nobody is going to stand rigid and take punches to the face. The point is to have enough neck muscle to control the acceleration of your head, as long as you accelerate 'slowly' then your brain wont sustain substantial impact. Like in a car, you can be traveling at speeds of over 100km/h for example, but if a car seat came flying at you and accelerated you from 0-100 almost instantly, you die, accelerate gradually, you ride.
I see what you mean. Neck strength does come into play. But I know from personal experience that locking your neck really boggles the brain when you get tagged. Although it is rare that someone can lock and just not move, doing so does probably overextend the muscles so I tend to agree with your rapid is death gradual is riding though processes.
So I would guess its a matter of control of muscular movement rather than a lack of movemnt or uncontrolld movement. Though I'd rather move uncontrolled and risk whiplash then let my gray-matter bounce around.