I was just talking about this on a family walk last night. In our great military wisdom (!), we decided that such a base was vital, as it appears a ground-based operation is going to be the only way to accomplish the stated goals in Afghanistan. A ground based operation needs close air support, all those attack helicopters and Warthogs and C-130 battlestations that don't have enough range to fly from Diego Garcia or carriers, etc. It doesn't look like the Taliban has the resources to mount any kind of effective attack on such a base (though if they tried it might be a good opportunity to deplete their resources). The sooner Afghanistan can be cleaned out, the better for all sides in the long run.