Some jobs just seem impossible to keep filled ... No matter what the explanation, it's clear that the sweet spot in al-Qaida management is No. 1 and No. 2. After that, job security seems only slightly better than that enjoyed by suicide bombers.
Slate speculates that U.S. spokesmen might be exaggerating the importance of the AQ figures we manage to clip, or maybe it's a Machiavellian plot to confuse the AQ rank and file, or else AQ just has a flat organizational structure in which everybody below Bin Laden and his henchman Ayman al-Zawahri are collectively the #3 leader.
Personally, I don't think it's so complicated. To slightly misquote Austin Powers: "who does Number 3 work for?" The two senior leaders of AQ keep themselves safely hidden away, so they need a fall guy to run operations in the field and take the risks. It's the same way many hierarchical organizations work; the Chiefs are inside pondering grand strategy while the Indians are outside getting picked off.