Ummm...one more thing:
The hip belt and chest strap, as well as bearing the load, keep the pack from shifting, which actually increases your agility.
Back to the backpacks I put up...do you think Mountainsmith and The North Face designed backpacks that limit your ability to "walk an edge."
Two key facotrs in any backpack design (blower or pack) are load distribution and stabilty, and I think the 580 does very well at both. It sure beats having the thing digging into to your shoulders, sliding around and bumping you in the donkey with every step like many blowers do.