The point of a deterrent is to deter. If you have to use it, it has failed. But if it's too big to use against anything except an existential threat, or indistinguishable from an all-out strike, then it can lack credibility against anything short of a strike meant to destroy your country.
Originally Posted by mrmalaya
The theory of a graduated deterrent is that the lower tiers deter smaller attacks, calculated not to trigger the launch of your strategic deterrent. ASMP was to stop someone taking the risk that he could, e.g., nuke a naval or air base (& an associated small city) without France daring to blow up Moscow in retaliation, by giving France the option to reply in kind to the lesser attack.
Juris praecepta sunt haec: honeste vivere, alterum non laedere, suum cuique tribuere.