It's extremely scary and sadly very possible.
I also think that the main reason they haven't used one yet is the fear and certainty of an overwhelming retaliation. They may have 5 or 10 bombs. Is it worth losing the Peshwar, Mecca, Medina, Western Afganistan?
Make no mistake a nuke used on the US removes all restraint. It is my guess that the response will be overwhelming, immediate and overnight.
The follow up invasion will be in the millions. Think Normandy, not Baghdad.
That's why they haven't used one. I fully expect it to happen, regardless of our policies, within the next few years.