Empires throughout history have fought in Afghanistan.
I think that is the key thing to understanding these people. Their cultural/genetic memory is imprinted with the idea that everyone else in the world can come here and kick their asses until they get bored and go home.
I think it's really their driving factor, and why they seem so unwilling (beyond cell phones) to pull themselves out of the 13th century. Granted, they realize (and use to tremendous effect) that the US will pay, and pay, and pay, for their ridiculous "projects" to make this a better place. They'll likely end up with the third world's best infrastructure by the time we leave--and given their insecurity, animosity towards each other, and general bat-shit insanity, they'll quickly devolve into internal warfare. Warlord vs. Warlord in a winner takes all cataclysm.
The winner of course, will get to take over all this wonderful infrastructure--and will likely tear it out and sell it to the highest bidder. Their whole economy (with the exception of the Poppy) seems to revolve around steal X from Y and sell to Z, who then resells it to X, and also get International relief agency Q to buy us more shit. Tremendous wealth awaits them just below the surface, in terms of mineral wealth (gemstones and industrial applicable mineral) and natural gas, and they have no desire to build any kind of road (or god forbid, rail) network to support it. And if someone were to build it for them, they'd end up just blowing it up, over a land dispute that started during when the cornerstone of Stonehenge was laid.
I don't really think of this place as a graveyard of empires--not the kind of place where empires come and are destroyed. It's more like the kind of place that empires come, see what it's like, realize that the only real solution outside of outright genocide is to dump prozac in every source of water and wait twenty years, realize that probably wouldn't work either, and then try to figure out the best way to just leave.
Unfortunately, we aren't there yet. I think we're getting closer to the "What's the point?" point, though. Staying here won't stop the spread of terrorism. Staying here won't make Afghanistan a stable democratic republic. Staying here won't remove the corruption, crime, or poverty. Staying here has no benefit to the US that I can see, long-term. The question that keeps popping into my head is "What's in it for us?" And I just can't seem to find an answer.
The second question of "is it worth it?" is even harder to answer. How do we quanitfy that? Is it worth it in terms of cost to the taxpayer? That goes back to "What's the point" and "How does this benefit the US?" What, exactly, are our strategic goals here? If being here is rooting out terrorists, we're playing whack-a-mole. If we're here to teach the Afghans to fight terrorists, then we're teaching Bear Grylls how to drink his own piss. They are quite capable insurgents, and counter-insurgents. They speak the language, have a vested interest in finding them, and have intel networks and a knowledge of the terrain that far exceeds our own. For the most part, I think they really just put up with us, knowing that for each unit they deal with, they only have to do it for a year, and that while we're here, they may as well get while the getting's good.