Josh Marshall is the latest prominent liberal commentator to come out against an all-out engagement in Afghanistan. Along the way, he makes this point:
But even if the Taliban again ruled the country, it's difficult to imagine that with our forces in the region and our army of drones, we'd have much problem raining down a ton of ordinance the first time they really put up their head.
I think it's a good one. In late 2001 we demonstrated for the Taliban the consequences of an al Qaeda attack originating from the safe haven they provided. Although the Taliban don't seem to mind endless warfare, it does stand to reason that, should he retake power, Mullah Omar won't be too keen about losing it again in the inevitable aftermath of another al Qaeda attack on the U.S. traced to Afghan soil. That deterrent effect could be enough to prevent a return to pre-9/11 sanctuary conditions. It's worth hearing more discussion of this, at least. (Pakistan, of course, is another matter....)
Update: Tim Fernholz reminds me of Steve Biddle's compelling argument, which is that Taliban control of Afghanistan means we lose the intelligence that enables us to take out al Qaeda leaders via drone attacks, as we do in Pakistan. Definitely a cause for concern.