Its a mystery to me as well except that I have heard it argued that the missile defense system would render the MAD defense void and open up the possibility that the person with the defense shield would feel they could opt for a first strike as they had negated the opponents ability to strike back.
"The purpose of fighting is to win. There is no possible victory in defense. The sword is more important than the shield and skill is more important than either. The final weapon is the brain. All else is supplemental." - John Steinbeck