|There have been (and still are) way too many tempting opportunities for Al-Qaeda to use a "fizzle" tacnuke device very effectively. If they had them, they would not still be sitting on them and accepting the massive risk profile that would ensue. Especially once Zubaydeh went down, the risk/reward quickly shifts to "use it or lose it."
Soviet tacnukes may have disappeared, but apparently not to al-Qaeda. As for the ISI, they''re ruthless but not stupid. Still, the "unstable Pakistan" scenario is real and requires equally serious contingency plans.