Seriously, why shouldn't they?
- Putting nuclear weapons in the hands of terrorists?
- Regional instability?
- A threat to the US or to the West?
- A bad precedent for other countries?
Debunking the above - and more arguments - below.
That ship has sailed - and not from Iran. The worst proliferators have been Pakistan and Russia. Their clients have been mostly other states, in the (wider) Middle East. Having a Middle Eastern State with an acknowledged nuclear arsenal would probably dry up a good chunk of the "demand" for clandestine nuclear know-how and materials, as a lot of that demand is linked to the perceived inequality with Israel.
Would the Saudis also want the nuclear weapon to balance the Iranians? That's a trickier question, to which I do not have a ready reply. But if you have Iran publicly a nuclear power, the same reasoning below could also presumably apply to the Saudis. I go into this in belowint, but I note this weakness in my argument here.
Putting nuclear weapons in the hands of terrorists?
Being a nuclear power is a very specific responsibility, and those that are have for the most part behaved accordingly. They have not being very good at limiting proliferation (that includes France and the US, along side Pakistan, Russia and China), but the transferees have each time been other countries (Israel, North Korea, Lybia, Iran) to which that same reasoning applies.
States understand the value of having nuclear weapons, and they apply the concept of proportionate response:
- do not use WMDs against a country which has no WMD;
- do not use WMDs first, unless to defend the country against an attack threatening vital interests;
- expect massive retaliation (including by WMD) by any country with WMDs which is attacked by WMDs that can be traced back to you (and nuclear weapons CAN be traced)
Iran is a State and will behave as a nuclear State if they are acknowledged as such. They will actually be forced to behave more responsibly if they enter the delicate balance between nuclear States. Support to terrorist attacks against Israel will be less necessary and more risky - becasue the rules above apply to Israel as well, and the first one would stop protecting Iran if they had nukes.
Israeli nuclear weapons are a major source of tension in the region. Restoring the balance and forcing both sides to acknowledge that balance (and thus each other's existence and rights) would probably be a lot more stable.
Having nuclear weapons forces you to have a doctrine with respect ot their use, and that includes very centralised control. We may not like the regime in Iran, but it is there and it is functioning, and they would certainly be able to manage their nuclear weapons and come out with the appropriate rules for use (these may be public, or privately communicated to other nuclear powers, or somewhat ambiguous, but the other nuclear powers understand that game well enough).
Acknowledging the nuclear option would force Iranians to provide a position viz. Israel, the other nuclear power in the region. This can only lead to an acknowledgement of the existence of Israel and its ability to defend itself (with nukes). As written above, small scale terrorist attacks against an also-acknowledged nuclear opponent will probably be a lot harder to justify.
It will force both sides to talk to each other - simply to avoid accidental launches or provocations from non-State actors. Diplomacy can only be a good thing in that context.
A threat to the US or to the West?
Iran is not a threat to the West today - except in so far as they can close off the Straits of Hormuz to oil tanker traffic, which is there irrespective of nuclear weapons and is actually a lot more effective as a WMD than a few nukes would be.
Nuclear powers work by the above rules. Any nuke attack on the West which can be traced to Iran will lead to massive retaliation.
Being an recognised nuclear power - and thus an unavoidable regional power treated as such by the West would probably do wonders to Iran's view of the world and their current belligerence would be much less necessary.
Having to discuss nuclear issues with the US, like with Israel, would lead to more diplomacy and dialogue, and probably fewer misunderstandings.
A bad precedent for other countries?
Well, as I wrote above about Saudi Arabia, I am probably on shakier ground here.
On the other hand, the Pakistan and India episodes have shown that they did not encourage any countries beyond those that were already contemplating nuclear weapons. Iran has been on that list for a while. I doubt that the Iranian precedent would change much to the North Korean situation.
The Arab world (even though Iran is not Arab) would probably see some justice being done in someone in the region having nukes as a balancing tool viz. Israel, and would have one major argument agaisnt Israel taken from them.
The rest of the world (especially those under the explicit or implicit US umbrella) would probably be happy to see any kind of balance work in that region, and not much change for their own local worries.
States are rational entities. They have a lot to lose and will thus control their nukes well, and so would Iran. Or is this all wishful thinking?
Have at it!