The (claimed) risk isn't that Iran would use nuclear weapons against the US in any official capacity. The risk is that Iran will simply leak nuclear weapons to a proxy terrorist organization (i.e. Hezbollah, Hamas, etc.) If you accept the US government's claims that Iran has used state sponsored terrorism as a political and social tool, then it follows that a nuclear armed Iran is a large risk to the United States.
I never said the claimed risk was that Iran would use nuclear weapons against the US - you seem to be attacking a straw man there. I believe the risk is that Iran would use nuclear weapons against Israel in retaliation to an attack on itself.
I don't buy Iran arming terrorists with nukes either. Both the US and USSR have used state sponsored terrorism as political and social tools, but never armed them with nuclear weapons. Without extremely plausible deniability, the repercussions of MAD would reach through if such weapons were ever used.
I think Iran sees nuclear weapons as its ultimate defense against continuing belligerent and threatening rhetoric from the US. And from my perspective as a neutral observer, it's hard to disagree with them. I think it's inevitable that Iran will get nuclear weapons, and the sooner that reality is dealt with on a rational basis, the better. If anything, the US should be bargaining with Iran to get a quid pro quo in return for acceptance of nuclear armament.
