Even if Iran does not use weapons directly, they can provide small nuclear devices to terrorist groups. We'll be seeing those within a few years. Iran has backed a number of terrorist groups (like Hamas) for many, many years.
But even for direct attacks from Iran - remember that Iran is full of many, many people who are basically innocents, rules by leaders that are almost wholly insane and do not care if their own people die.
If Iran were actually ruled by leaders who thought that the 12th Iman would return and kiss their own crispy foreheads as well as the crispy heads of their grandchildren, they could have simply rained down conventional missiles on Israel and gotten that response. Why not go to heaven sooner, rather than later?
You do not get the full benefits of membership in the nuclear club by acting crazy. The crazy talk is not actually helping North Korea, it is only causing China to see the day to whip North Korea into better behavior is coming sooner than they expected.
Held onto as a last resort, nuclear weapons are a positive asset. Employed recklessly, nuclear weapons are a liability -- because once you have been proven to be completely reckless, the entire nuclear club with see the reasonableness of scraping your sorry nation off the face of the earth with nuclear fire. Where is the fun in that?
Furthermore, letting nuclear weapons out of your fortified safe places is dangerous. Crazy terrorist groups do crazy things. In the case of Iran, the ME is filled with enemies. Can you be sure it will not be captured and used on YOU? Absolutely sure? Is this a risk that is so worthwhile? Mossad is likely to figure out where the bomb came from -- there are only a few possible sources. So your plausible deniability game does not guarantee anything.