Wright links to an academic paper (When Heads Roll: Assessing the Effectiveness of Leadership Decapitation) - a great read - which he summarizes this way:
There’s no way of answering this question with complete confidence, but it turns out there are some relevant and little-known data. They were compiled by Jenna Jordan of the University of Chicago, who published her findings last year in the journal Security Studies. She studied 298 attempts, from 1945 through 2004, to weaken or eliminate terrorist groups through “leadership decapitation” — eliminating people in senior positions.
Her work suggests that decapitation doesn’t lower the life expectancy of the decapitated groups — and, if anything, may have the opposite effect.
That conclusion seems right to me. I've lost track of how many times it was announced that we had just killed the Number Three al-Qaeda leader in Iraq, Afghanistan, or Pakistan, but it sometimes seemed like we were killing the al-Qaeda S3 (see page 36 of this document if that term is unfamiliar) about every month or two. The Number One guy stays well hidden, of course, and I guess the Number Two is too important to expose himself much, so it's the poor Number Three who has to run operations in the field and ends up catching a missile.
Do all those sudden terminations of senior staff officers degrade al-Qaeda in the end? The numbers seems to say they do not.