But if 60% of the population is relatively immune, it makes it harder for an epidemic to get out of control, and creates a firewall of high-immunity which would aid the effectiveness of a quarantine if there is an outbreak.
It also means that if the 15 million doses on hand can be effectively diluted to 75 million (the CDC's stated goal), the real target population for the vaccine is down to around 110 to 120 million.
We could make giant strides toward "herd immunity" in a hurry, in the event of an emergency.
We will need more than 60% of the population to be immune for herd immunity to stop an epidemic....although I do agree with your point. Even with significant immunization for diphtheria, we still have occasional outbreaks. No epidemics though.