An interesting point, but I think your problem is that you’re approaching this as if Al Qaeda were rational terrorists, like we had in the good ol’ days. It used to be that, say, the IRA or Eta would use terrorism in pursuit of a specific goal- sure, it might be a big one, but it was generally achievable (British out of Ireland; Basque independence), and the terrorism was designed to draw attention to the cause, to try and show the “strength” of the underdogs (terrorism is only ever used by underdogs; it’s called something different when the powerful do it), and to maybe, just maybe, force the foe to the bargaining table.
But the problem is, Al Qeda isn’t like that; sure, they do have a specific goal, but an ultra-Orthodox Wahhabi-based Islamic caliphate ain’t happening any time soon- furthermore, the people that Al Qaeda is targeting* aren’t the ones who can achieve it. What Osama and his cronies act out of is spite, pure and simple. Without a single, achieveable goal (like, for example, driving the Russians out of Afghanistan), their terrorism splinters into no more than serial murders, driven by no more logic than a desire to hurt others.
If Al Qaeda did grab a nuclear bomb, what would they do with it? I assume they’d do whatever they found most emotionally satisfying, not what would help advance their (lost) cause. And what would that be?
Well, actually, I think they might nuke Mecca. I’ll admit to not being anything like an expert on any form of Islam, but according to my daily newspaper, ultra-orthodox Muslims in Mecca are actively destroying religous sites (aided and abetted by town planners) to prevent anything that looks like idolatry- for example, the tomb of the Prophet’s wife has been burned, and anyone praying there is supposedly shooed away by “Security Police”. If someone like Bin Laden got his hands on an a-bomb, what would be more satisfyinng than to send a powerful symbol to the most “corrupt” country in the Middle East, drive idolaters from the site forever (well, at least those not wearing radiation suits), so on, so forth.
On the other hand, maybe everything I just said is bollocks, so please, feel free to rip me to shreds.
*Yes, I know that Al Qaeda attacks other Mulsims, not just the Western states; yes, I know Al Qaeda is active in Saudi Arabia, where any caliphate would have to start. But they are loudest about their attacks on Americans et al.