Where do the slings put the weight then? You only have one support from your upper body to your lower, and that's your spine. Seems like a backpack would even the load left-to-right, but not back-to-front. A sling would screw up the left-to-right distribution, but could be adjustable to even out the front-to-back. A larger backpack can be adjusted to put weight directly on your pelvic bones (albeit that weight is skewed to the back of the pelvis) thus leaving a smaller load on your spine. Maybe one asymmetric weight distribution is better than the other???