Keeping the Arab world from building their own nuclear weapons has long been contingent on Iran not having a nuclear weapons program. It only benefits the US to the extent it prevents the situation where half the countries in the Middle East having nuclear weapons.

Good lord, it benefits far more than just America if the broader middle east doesn't enter into a rapid nuclear weapons proliferation stage. Iran is considered to be a very serious enemy throughout much of the middle east. A nuclear armed Iran is a very bad idea.