Though there is no shortage of tensions in the Middle East, I don't know that Iran has any avowed enemies as nation-state neighbors, so if I were a sane Iranian, the last thing I'd want is possession of a weapon that by your logic would encourage neighbors to do the same thing. On the other hand if I were an Iranian dictator at risk of being tossed by sane Iranian citizens, a weapon that scares everyone should my government become unstable is exactly what I'd want.
Meaning what, they all have roughly the same behavior? Then Cuba, Zimbabwe, The Sudan, the Palestinians in Gaza are all also free to obtain nuclear weapons? Aside from greatly increasing the chance of a country to country war, such a world would have a great increase in proliferation! That is, the world would have much more weapons grade nuclear material, bomb designs, and people with expertise in both, in the hands of countries with little experience in securing it all, increasing the risk that such material or people fall into the hands of an Al Qaeda type.
I add last that I don't see, on balance, today, that a military attack on Iran to stop it from getting a weapon would be wise, effective, or warranted, but I don't come to that conclusion from pretending nothing bad can happen should Iran acquire a weapon, as Ron Paul suggests.