For our great ancestors, a brain with increased excitability allowed them to:
1) Find food
2) Avoid danger
3) Care for the children and tribe
So the hypervigilant person would be the one, for example, to smell that food is going bad before anyone else can; s/he would be the one easily awakened and alert to danger earlier than others. They would be the care-takers of the "tribe", so to speak.
This would be an honourable role; one that, not surprisingly, is not appreciated by the hypervigilant individuals in our circles.