Pelosi Air Travel

All the wingnut hubbub over Pelosi turned out to be nothing more than wingnut hubbub when it was revealed that Hastert used the military flights as much or more than she did. What I don't get is the rationale for this post-9/11 policy. The justification given is that we live in a world of terrorism, Pelosi is third in succession, and what if the White House is attacked or something. Yet... I still don't get it. What scenario is being envisioned where Pelosi flying with extra security measures is necessary? Yeah, she's President if the VP and POTUS are both incapacitated. Why does giving her military travel clearance before this is an issue seem logical?