HOW SPECIFIC DO WE NEED TO BE?

According to Bush, quoted in the WaPo, the controversial and now famous PDB security breifing, "said nothing about an attack on America."

The briefing reveals that in 1998, US intelligence learned that Bin Laden wanted to hijack a US aircraft. "FBI information since that time [and, as we have learned from elsewhere, strong signals in the summer of 2001] indicates patterns of suspicious activity in this country consistent with preparations for hijackings."

I'm wondering what is the value--in terms of this information--of being more specific? It seems that the government would focus its preventative actions on stopping a hijacking of a US aircraft irrespective of what analysts thought terrorists might intend to do once they hijacked a plane. It makes no difference whether terrorists want to hijack a plane to use as a bargaining chip against their imprisoned compatriots or to use to crash into really big buildings in New York and Washington. Either way, we would want to stop them from wresting control of the planes.

How would specific knowledge that planes were to be used as missiles change the government's response? Would Bush have installed bumpers on the Trade Center towers? Would he have pressed to speed up the development of his National Missile Defense system?