Restricting Iran's ballistic missile program would further lengthen their breakout time
Finally, missile restrictions would slow down Iran’s capacity quickly to field capable offensive nuclear forces in the event that it ever reneges on an agreement or merely waits out any time-limited provisions. In terms of such “breakout” potential, one of the gravest concerns about any agreement allowing the Iranians to preserve a robust nuclear reprocessing capability is that, without even needing to cheat, it could allow Iran to tiptoe up to nuclear weapons threshold status. In other words, as long as Iran is allowed to maintain an enrichment program for peaceful purposes, it will retain a latent capability that could quickly be put to use to produce weapons. Once the centrifuges start spinning to enrich nuclear fuel to weapons grade, it is simply a matter of time, more or less, depending on the number of centrifuges.
As the North Korean case shows, it is easier to develop nuclear explosives than reliable long range missiles capable of delivering them. Whereas the North Koreans have conducted successful nuclear explosives tests (albeit with mixed results), they have not yet mastered an intercontinental missile capable of hitting the continental United States, nor the ability to deliver a nuclear warhead on any range missile. Iran likewise is still pursuing long range missile capabilities and has yet to develop missiles that can hit the United States or even Western Europe . A ban on any further Iranian development, testing, and production of nuclear-capable ballistic missiles would lengthen the timeline between an Iranian decision to renounce an agreement and its ability to deploy nuclear armed missiles, in particular any that could threaten the United States and its key allies outside of the Middle East.