That is not true at all, a nuclear weapon is a highly useful deterrent, especially against conventional attacks. Ask Kim Jong-un about it.
I was under the impression that the true deterrent there was hardened and decentralized conventional artillery able to do significant damage to Seoul, since we're pretty sure North Korean nukes will work as well as their cure for MERS, Ebola, and AIDS.
Ideally you want multiple deterrents, of course.
As to the chances of the nuke working, well, you gotta ask yourself, do you feel lucky, punk? X-/
Edited to add: We are discussing here whether Iran wants nukes. Therefore what is relevant is that the Kims wanted nukes, even though they had the artillery-can-reach-Seoul deterrent already.
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