The world would be wise to keep al Qaeda alive, paradoxically enough, for security reasons. Like it or not, keeping a battered al Qaeda intact (if weak) is the world's best hope of funneling Islamist fanatics into one social network -- where they stand the best chance of being spotted, tracked, and contained. The alternative, destroying the terrorist group, would risk fragmenting al Qaeda into thousands of cells, and these will be much harder to follow and impossible to eradicate. It's the counterterrorist's dilemma, and the only real choice is the least unsavory: Al Qaeda must live.
Killing off al Qaeda would do little to reduce Islamist terrorism. It would only make the world of terrorism more chaotic.
De Las Casas concedes that keeping al-Qaeda around has obvious risks--they are terrorists, after all. But could the dangers of a destroyed al-Qaeda outweigh the benefits?