If we're talking national security we have to talk more than just destroying random terrorists, it's about nation building over there. Pure, plain, and simple

Afghanistan isthe most sought after nation in history, and there's virtually a civil war right now to try and oust the Taliban. As long as they're in power not only are Americans probably directly in danger but certainly countries such as Russia and other parts of Europe with huge Musilm populations as well.

It would be ridiculous to leave these people in the 14th century when they have the potential to be the most modernized and forward looking country of this important region.