Here are some of the benefits:
Ay, there's the rub. Everybody knows why the Israeli right will fight an US-Iran agreement like the plague -- as Iran as an "existential threat" is the ideal pretext to change the debate from the real issue; the occupation/apartheid regime imposed on Palestine.
The price of oil and gas from the Persian Gulf would go down;
Washington and Tehran could enter a partnership to fight Salafi-jihadis (they already did, by the way, immediately after 9/11) as well as coordinate their policies in Afghanistan to keep the Taliban in check post-2014;
Iran and the US share the same interests in Syria; both want no anarchy and no prospect of Islamic radicals having a shot at power. An ideal outcome would balance Iranian influence with a power-sharing agreement between the Bashar al-Assad establishment and the sensible non-weaponized opposition (it does exist, but is at present marginalized);
With no more regime change rhetoric and no more sanctions, the sky is the limit for more trade, investment and energy options for the West, especially Europe (Iran is the best possible way for Europeans to soften their dependence on Russia's Gazprom);
A solution for the nuclear dossier would allow Iran to manage civilian use of nuclear energy as an alternative source for its industry, releasing more oil and gas for export;
Geopolitically, with Iran recognized for what it is -- the key actor in Southwest Asia -- the US could be released from its self-imposed strategic dogma of depending on the Israeli-Saudi axis. And Washington could even start pivoting to Asia for real -- not exclusively via military means.
As for the House of Saud, such an agreement would be nothing short of Apocalypse Now.
I'm just a moderate killer
It starts with Syria. Everybody now knows that shadow master Bandar bin Sultan, aka Bandar Bush, has been fully in charge of the war on Syria since he was appointed Director of National Intelligence by his uncle, Saudi King Abdullah.
Bandar is taking no prisoners. First he eliminated Qatar -- the major financier of the so-called Free Syrian Army (FSA) -- from the picture, after having a helping hand in Qatar's emir, Sheikh Hamad, deposing himself to the benefit of his son, Sheikh Tamin, in late June.
Then, in late July, Bandar spectacularly resurfaced in public during his now famous "secret" trip to Moscow to try to extort/bribe Russian President Vladimir Putin into abandoning Syria.
Notoriously, the House of Saud's "policy" on Syria is regime change, period. This is non-negotiable in terms of dealing a blow to those "apostates" in Tehran and imprinting Saudi will on Syria, Iraq, in fact the whole, mostly Sunni Levant.
In late September, the Jaish al-Islam ("Army of Islam") entered the picture. This is a "rebel" combo of up to 50 brigades, from supposedly "moderates" to hardcore Salafis, controlled by Liwa al-Islam, which used to be part of the FSA. The warlord in charge of Jaish al-Islam is Zahran Alloush -- whose father, Abdullah, is a hardcore Salafi cleric in Saudi Arabia. And the petrodollars to support him are Saudi -- via Bandar Bush and his brother Prince Salman, the Saudi deputy defense minister.
If this looks like a revamp of the David Petraeus-concocted "Sunni Awakening" in Iraq in 2007 that's because it is; the difference is this Saudi-financed "awakening" is geared not to fight al-Qaeda but towards regime change.
This (in Arabic) is what Alloush wants; a resurrection of the Umayyad Caliphate (whose capital was Damascus), and to "cleanse" Damascus of Iranians, Shi'ites and Alawites. These are all considered kafir ("unbelievers"); either they submit to Salafist Islam or they must die. Anybody who interprets this stance as "moderate" has got to be a lunatic.
Incredibly as it may seem, even Ayman al-Zawahiri -- as in al-Qaeda central -- has issued a proclamation banning the killing of Shi'ites.