Clayton asks, What, Exactly, Is The Purpose Of Blowing Up A Bunch of Airliners?
A couple years ago I was talking with a guy in the CIA. He was a manager over, among others, a bunch of psychologists. What he told me was that "these people" think differently than we do. "Perhaps more differently that we can think." Most important about this is that we understand what to do and/or say to get them to do the things we want them to do, next week, next month, and next year. And these means to these different goals may be in conflict with each other.
Now for my speculation. We probably think so differently that they have difficultly thinking like us as well. They probably are doing what they think should cause us to do what they want us to do, but as Clayton points out, it is going to make us more determined to capture and/or kill their extremists. It's sort of a messed up communication channel.
Another possibility is they just want to kill as many infidels as possible. They are just delivering on the "convert or die" offer from their leaders. If you look at the kill ratio when our military goes up against them it's probably 1:20 to even 1:100 in our favor. The numbers are against them in that situation.
Ry and I once did an informal study of kill ratios for various "occupations" such as military, mass killers, serial killers, terrorists, etc. This was a little different than your normal ratio of dead on one side to dead on the other side. In this case what is the ratio of people involved on one side to the number of people dead on the other. So if Ted Bundy acted alone, which I think is most likely true, and he killed 43 (or maybe 143--who knows?) then the kill ratio there is 1:43 (or 1:143). In the Oklahoma City bombing 168 were killed and there were probably two people involved for a ratio of 1:84. In 9-11 there were 19 (plus some organizers and support people) who killed something like 3500 people for a ratio of (on the high end) of 1:184. In the case of the military dropping a nuke you have to include all the people that helped make and store the nuke and it's delivery system, as well as the people in the chain of command who delivered the nuke.
It's not difficult to concluded that 1:200 is about the upper limit.
Therefore, if the goal is to kill as many infidels as possible losing one "martyr" per airplane full of infidels is pushing the limit of what is the "current state of the art" in dealing out death.
But who can know if that's right? My guess it's going to be difficult for us understand even if they were to explain it to us.