Why?
I have heard this line of reasoning time and time again--chiefly (though not entirely) from devotees of the "realist" school of foreign policy--and it has never yet made even a scintilla of sense to me.
The logic (such as it is) seems to go something like this: If only the US Army were not otherwise engaged right now, it would be available for combat in Iran. But that begs the obvious question: Why would we wish to use this branch of the armed forces against Iran?
Many of us believe that we would benefit by engaging Iran militarily now, rather than later; and, moreover, that any thought of a third, more antiseptic alternative--say, regime change--has all the realism of a Hans Christian Andersen fairy tale. But even all-out war with Iran need involve no more than the US Navy and Air Force.
Why would we want to occupy that country with ground troops?
Disinformation?
I also wouldn't want a regime changed Iran to have nukes either. It would be a weak government in a nasty neighborhood. Iran, whatever its government, should not be allowed to have nukes.