For those who do like shoulder bags, consider this. The County Comm BOBs -- which I like -- have a thing strap with a small pad. Not much support, and serious wight over the long term can physically damage your shoulder.
But true messenger bags or the Maxpedition Gearslingers have broader shoulder straps designed to cushion for the extra weight. A small improvement, but btter than nothing.
Still, the symmetry and doubled loadbearing padding of a backpack will tame loads over fifty pounds, which a shoulder strap just isn't made to handle.