Makes sense, unless they only have 1, 2, or 3 weapons (which is much more likely than 40). Why waste them?
Because they're being hounded left and right and the FBI could arrest them at any point and a nuke used is better for them than a nuke unused and wasted. There's no way Al Qaeda has nukes.
Ok, I'd like to revise that. It is conceivable, if unlikely, that they could have obtained a device from Pakistan. But that (1) would be large and unwieldy (not a "suitcase") and (2) would be at most one bomb, not 20.
The panic they could cause would almost be enough. So, using Occam's Razor as a gauge, why haven't they done this yet if they have them? Somewhere - anywhere? Talk about getting leverage.