There are a lot of things going on here and everyone is speaking the same language, but maybe different dialects.
I've studied pack design for a long time, and have been fortunate to own a lot of different high end packs before starting my own company, Paradox Packs.
Comfortable load carriage is a function of the hipbelt, frame rigidity, frame shape, and frame height. I don't think you can talk about one piece of the system without the others, because they are an interdependent system.
On the hipbelt I believe that to be comfortable at loads of 10-60 lbs many different designs work well, and lumbar pad designs work efficiently at these loads because they are designed to effectively give very good contact on the lumbar and the fronts of the two iliac crests.
However, at heavy weights the lumbar pad belt is a failed design in my experience because it deforms and slips, lowering the frame height relative to your torso, and in some cases slipping so much it interferes with free movement of the legs.
I believe that a properly executed full wrap belt is much superior at heavy loads, and equal at light loads.
Moving on to frame rigidity, the level of stiffness in the frame required to support the load increases with the weight and awkwardness of the load. So, you can get by with a frameless pack for light loads, but for anything over 15-30 lbs you need some support, and for heavy loads you need a very stiff and strong frame.
Frame height is more important than load lifters, as load lifter function is simply a product of frame height.
The photos Nick posted illustrate this perfectly. His packs have no load lifters, BUT the frame is slightly above his shoulder level, thus the shoulder straps are simply snugging the pack up to his back instead of supporting the load with his shoulders. This type of system is fine for medium loads.
My preference is to have a frame that is 2-4 inches taller than shoulder level for most uses, a frameless pack for daypack use, and a tall frame for heavy loads. I prefer to have load lifter straps.
Here is why:
- Shoulder straps serve three main purposes in my opinion:
1. Provide handles for shouldering the pack, and support the pack while you adjust the hipbelt and get it tight.
2. Snug the pack to your back while you're wearing it.
3. At heavy loads, I like to alternate carrying more weight with my shoulders, and carrying less weight with my shoulders.
A pack with a properly fitted shoulder harness with a frame above shoulder height and load lifters will accomplish those functions at all load ranges, IF you have proper frame rigidity for the load, frame height for the load, and a hipbelt that does not slip.