Yeah, before I knew any better all we Boy Scouts had packs with just shoulder straps. Not terribly pleasant, but not built to today's standards, either. Not only do the shoulder blades and clavicle take a beating, the pack bounces on downhill stretches.

A relatively light, very well fitted and properly padded pack can be okay, and a sternum strap probably helps too, and I still recommend a waist band to prevent bounce and shifting. It need not be weight-bearing.

Good luck!