The higher ups will tell you that it is a "layered response." The old swiss cheese debate that was present a few weeks back. IF you take a piece of swiss cheese and put another one on top of the first one, the holes (security leaks) dont line up and the more "slices" you add the more holes you plug. So if a shoe bomb is a threat, the shoes are screened, (one layer of cheese.) Then ignition sources are removed (another layer of cheese) in the hopes that if you miss one threat, the other is found. I know it sounds cheesy! Sorry I couldnt refuse.