Which is why when I went hiking, I attached a couple of home-made carriers to the front straps of my rucksack. One to carry a water bottle, and the other to carry a camera. Having some of the weight on the front (rather than all of it on the back) helps. There's (quite old now) research that shows that people in Africa who carry immense loads on their heads actually have a more efficient walking gait than people who carry loads on their backs.
I suspect the amount you can "carry" in your body (ie. not including the amount you'll urinate 1 hour later) is in the order of 1-2kg tops, so ease of carrying isn't noticeable.
1-2kg can be a big fraction of the total weight an ultralight backbacker packs. I'm by no means an ultralight backpacker and my backpacks are usually around 10-15kg. A 10% saving is definitely noticable on longer hikes.
