Why is this being modded down? If you don't consider the worst case scenario in strategic decision making, you're a fool. Especially when the worst case is famine, plague, etc., not "my startup crashes and I move back in with the parents".
The weight given to an outcome in considering it in decision making would be the probability x magnitude (+ or -). So on your drive home the risk of death might not affect your decision to make the drive, if you had to drive across Johannesburg or Kuala Lumpar, you might consider taking a cab.
So even while the probability that the Country A would survive a nuclear first strike with the capability to retaliate was quite low -- the magnitude of the outcome from its doing so was enough to dissuade Country B from taking that chance.
To the other commenter below -- just google 'prisoner's dilemma' or 'Nash equilibrium' for some sources to get you started on game theory.
Note: I'm not claiming to be an expert on game theory by any means. I'm recounting the Mutually Assured Destruction example from an economics class's subsection on game theory and would be happy if someone wanted to prove me wrong or elaborate on the subject.
No, the worst case scenario is you crash and are alive but trapped... At night, the ice weasels come.
People should be informed about the issues in a manner that is as honest and realistic as possible, particularly when the uncertainties are so great.