I do agree that the biggest threat from nuclear proliferation isn't that some established nation is going to launch on us or any other country.
The threat is that some country is going to "lose" a warhead. The US has lost at least one warhead (that I've read about, I'd bet they've lost more). It's not inconceivable that other countries might lose warheads, whether by accident, theft, or convenient cover-up for an arms sale...and the more volatile countries that have them, the more likely that is to happen.
Tops on my list of concern would be Pakistan followed by North Korea. Iran doesn't really concern me, but I don't care if Tel Aviv gets nuked by some nut in the worst case. Really, though, I wouldn't consider Iran all that likely to turn something over to a client organization with both Israel and the US gunning for a war. I think Israel is also most likely to use nukes in a preemptive strike.