Iran getting nuclear weapons is not seen as dangerous because Iran would be another nuclear power, but because in that part of the world everything, including government nuclear weapons secrets, are for sale. That's how Pakistani nuclear weapons scientist Abdul Qadeer Khan made himself rich, selling nuclear weapons technology in the 1990s, after stealing some of them from Europe in the 1970s. Kahn eventually got caught. But because he was a national hero for leading the development of Pakistan's nuclear weapons, he got put out to pasture, with his money. There's a growing market for nuclear weapons technology, and now everyone knows that you can sell that stuff and get away with it.
Iran is particularly worrisome because the Islamic conservatives running the country have shown an impressive lack of scruples when it comes to buying and selling weapons and related technology. Iran has a network of smugglers throughout Europe and North America. Most of these people are freelancers, but the Iranians pay well, and use their network to quickly and quietly move stuff, and people, around. In this way, the Iranians have managed to keep thirty year old, American made warplanes and missiles, in working order. A network that can move aircraft parts, can also move bits of nuclear weapons technology.
North Korea has a similar network, although not as efficient as Iran's. Iraq and Libya used to have such networks. Iraq's was quite good, until it collapsed with the removal of Saddam Hussein in 2003. Libya voluntarily got out of the business in 2004, preferring to make peace with the rest of the world, and not have nukes, rather than risk ending up like Saddam.
But Iran moves right ahead, and shows no intention on giving up its outlaw ways once it has a working nuke. For the Islamic radicals that control the Iranian nuclear weapons program, getting more nuclear weapons programs going, and making some money off it, just makes it more likely that an anti-American terrorists will get, and use, a nuke from someone besides Iran. That way, America gets what Iran wants it to get (a nuclear terrorist attack), and the nuke cannot be traced back to Iran.