"I also think that Ray Jardine is plain wrong assuming that everyone would be comfortable sans hipbelt...."
I agree - up to a point. Ray's shoulder straps have always been marginal - even to the point of failure, at least until the Breeze was redesigned with a different attachment strategy.
Skepticism about shoulder straps is sort of understandable. There are lots of really poor shoulder straps out there -- more so now that folks rely on hip belts. Shoulder strap design has atrophied. However, saying that hiking without a hipbelt is impractical is sort of like saying it is impractical to hike in sandals when you have only seen flipflops.
I don't use a hip belt, and haven't for some time now. But I design my own shoulder straps: wide, contoured, shoulder-wrapping, not very thick,lightly padded straps. They work for me up to the 35 pound range without shoulder pain or damage. I will admit that a period of conditioning is necessary as with many backpacking related things - such as boots or (dare I say it?) hip belts. I also believe that the adjustment period is not unreasonable or burdensome.
A lot has been lost by over-reliance on hipbelts. For example, it might seem reasonable to assume that the hip belt is needed to keep the pack from sliding off your back when climbing with your body horizontal and sideways. With most packs that may be true, but with well designed shoulder straps, the straps will keep the pack on you back. For more extreme climbing, a simple waist strap is sufficient to stabilize the load.
Also lost is freedom from the constraint of the hip belt... natural hip movement and natural stride.
Finally, the belt restricts movement of the satorius and quadracep muscles which are fully flexed when climbing and get a serious workout when ascending. In severe cases, the belt can cause tendonitis where these muscles attach to the pelvis. It is not unusual for regular hikers to complain of pain in that location. It's the belt, but you can't do much about it since the pack won't usually work without it.
The problem is, it's not usually practical just to unbuckle your hip belt to test whether you would be comfortable without it. What you get is an unfair test because the belt is part of a system on most packs (any pack that is any good, at least), and the shoulder straps are not usually designed to work alone. Several problems may arise. Often the belt places the pack at an angle to the back that makes the straps uncomfortable or unstable. In some cases, you can remove the belt entirely and have a useful pack, but not in every case. Often an attached lumbar pad makes the pack seem unstable. And the pad may rub uncomfortably when not held to the back by the belt. Worse, you may find that the shoulder straps are simply uncomfortable when carrying the full burden of the pack. This does not prove that using a pack without a hip belt is inherently uncomfortable; only that a particular pack with a particular load and particular shoulder straps is uncomfortable.
While the lava was still cooling, pack makers asserted that hip belts were useful for loads greater than 35 pounds. Of course, this was in the early days right after they developed the padded hip belt. At the time, I was packing 50-70 pounds with wilderness training programs, and took to hip belts right away. With those weights, they were great. On level ground. For steep ascents, they were worse than useless, and I shed them. To give a modern example you may have noticed, the "croos" at the huts in the White Mountains (at least the several I have seen in action) prefer to pack cases of canned goods on Trapper Nelson frames. No belt. See the comments about leg muscles.
About the rotator cuff. It is almost inconceivable that shoulder straps would initiate a rotator cuff tear. However, it is very likely that poorly designed straps could exacerbate preexisting damage. My significant other, the indominable Phriteaux, has had both rotator cuffs reconstructed - and the damage was due to cycling accidents, not backpacking. The rotator cuff injuries didn't stop her backpacking - before or after surgery. She does not use a hip belt either.