This is similar to what you've said, but I'd say the issue is more that it becomes more advantageous to use them if the same people who would normally levy sanctions against you have all disarmed. Currently there's little risk of North Korea actually using nuclear weapons. If they could do so without nuclear retaliation, it becomes a much more likely scenario. Even if banning nuclear weapons makes it much more difficult to obtain them, the consequences of someone obtaining them become much worse.