The theory usually is that, even though such a party can be reasonably sure that its cheating would not be detected, it will, nonetheless, be deterred from doing so by the threat that other signatories will take unpalatable retaliatory steps.
CENTERFORSECURITYPOLICY: Center for Security Policy | Submitted Testimony Of Frank J. Gaffney, Jr. Before A Joint Hearing Of The Senate Governmental Affairs Committee And The Permanent Subcommittee On Investigations