I have a co-worker who packs llamas for summer trips in the Rockies. He says they are something to carry your backpack. They can sustain about what an average well conditioned hiker can. Since many tourists are not well conditioned, the llamas are needed.

I can see how this could be very helpful for a soldier when the circumstances are right to employ it, since the soldier wouldn't arrive at his objective as fatigued. I don't doubt llamas have their place but they're not exactly like the Roman practice of one mule per tent group either.