Two of the best reasons not spoken for a war with Iran are that it would bring in China and push out Russia.
The latter first: Iran has been transforming into a Russian client state, and this relationship is enormously profitable to Russia. By supporting the Islamic Republic, Moscow is able to distract Washington from more important goals throughout eastern Europe. The fate of the soft revolutions against authoritarianism and the expansion of Europe as far east as possible simple matter to us far more than does the particular fate of Iran, or even the Shia generally. As long as Moscow is able and willing to provide Iran cover, our important work in Ukraine, and Georgia, and beyond that in Belarus and Kazakhstan, is set back. If Iran in chaos is the price that needs to be paid for expanding the European Care and crippling Russia’s ability to cause mischief, then those benefits alone mean a positive ROI (return on investment).
The former last: One of the many reasons that America had trouble expanding the coalition of the willing to include Iran and China is that the Asian states are accustomed to free-riding of American efforts in the Gap (the Muslim world and Africa). Unfortunately, much of the hard work in shrinking the Gap relies less on stealth bombers and more on boots on the ground. American labor is simply too expensive to allow Washington to field a 200,000 man army a quick and successful Iraq stabilization may have required, and similarly too expensive to do much good throughout Africa. Critics of strikes on Iran often say that such a war would invite increased attention to the third world from China and India. I say good. We need the powers of the New Core as partners. If the Iran War enables that, then the struggle is worth it.