Due to an old neck injury I am unable to carry any weight, at all, bearing down on my shoulders. I adjust the pack to support the weight on my hips, with the shoulder straps set to have weight only pulling back, supported on my collar bones.
I can, if necessary, carry my whole pack weight (Luxurylight framed pack, six day trip) without the hip belt, if I over tighten the shoulder straps so all the weight is on my collar bones. It's not comfortable but, when the buckle failed on my pack, I was able to carry on for three more days.
What I have noticed, on the trail, is hikers whose hip belts have fallen below the iliac crest. Their packs are not riding correctly and they often complain of discomfort. That reminded me that last Saturday, in the Berkeley, CA REI I pointed out a customer, trying on a new pack, to a staff member and noted that the pack was about two inches too short for his torso length. The staff member agreed and suggested a longer pack to the customer. Too short a pack puts too much weight on the shoulders and does not allow the hip belt, or the shoulder straps, to do their job properly.
Just some information gleaned from leading workshops for beginning backpackers, since 1987. After missfitting boots, missfitting backpacks cause the most grief. Oh, then there are exploding stoves. LOL, but that's another story.