Whether you agree with it or not the main reason the US is prepared to negotiate with the moderate elements of the Taliban is so that when they do leave (2015? That the expected date?) there is less likelihood of a power vacuum of sorts.. what with a weak president in Karzai with his corrupt government, police and ill-prepared army up against the Taliban while still also trying to appease the various warlords and other factions.
In all probability, the country will descend into another civil war with the end result possibly an entity enforcing its strict interpretation of Islam, whose default position is outright hostility towards the US, the welcoming of Al Qaeda or other factions to breed as well as a contagion effect on Pakistan and beyond. So yes, the US would likely be forced to intervene once again in years from now to protect it's assets in the Middle East.