My take: Internal frame packs were only intended, really, for those who ventured off-trail into brush, or needed the extra stability of a load closer to the core (skiers, climbers), or possibly lower wind resistance above tree line. I've heard many of the old externals are actually lighter than some of the huge seven-pound internals you see. Makes sense--why pay for and carry fabric to carry something that can be lashed as it is to a frame? (Many internal frame packs are filled with stuff sacks, anyway.)
I think what happened in the eighties is that internals got popular among the on-trail crowd because it made you look more "serious". After all, the folks coming down from the summits and couloirs, all scratched up with ice axes and ropes, were using them, so they must be better packs. And they look even sexier with a carabiner or two attached. Your Nalgene looks much better hanging from a carabiner, after all. So the manufacturers started supplying them with more straps, "daisy chains", and two ice axe loops are better than one!
There's nothing "wrong" with an external frame, especially on the AT. Bring the right tool for the job.