I think you hit the nail on the head, with the key expression being "workable". While I'm skeptical of this story in general, it IS possible that AQ does, in fact have one or more nuclear wepaons, suitcase-sized or otherwise.
However, the very technical sophistication that allows subcritical masses of fissionable material to be used as weapons also makes them expensive and difficult to maintain in combat-ready condition. Because they usually require such short half-life components as tritium, the triggers for such tiny nukes tend to degrade rather quickly.
AQ may very well have nukes, but lack the ability to detonate them. To be sure, they would (if this is true) be seeking triggers for these devices, but triggers are very weapon-specific, and, unless the ones they obtain were made for their specific models, they would be extremely difficult to adapt to the weapons they have.
That may ultimately inspire them to either try to pool the fissionable material in their weapons to make a crude, low-yield device, or parcel it out for dirty bombs.
In any case, while it's hard to be certain whether or not AQ has nukes, it's not hard to imagine them acquiring radioactive material, even if it's from X-ray machines.
That's why I think U.S. agencies are mainly concerned about dirty bombs for now, and are trying to head off terrorist acquisition of modern, functioning nukes by going after regimes such as Iraq, Iran and North Korea, while keeping Pakistan firmly under our thumb.
Yes, if they thought like you do, but they do not. They are following their own strategy, which calls for hitting us with one thing at a time, over and over again, maybe gradually increasing the heat as well. Whether it will actually "work" or not (and I believe that it will not) is not the point -- if they think this will work, this is what they will do, whether it makes sense to us or not.