Sometimes I dive off the beaten path and have to lug the gear across long paths that have no roads. It's about weight distribution for me during the walk. I am not sure how much lighter a carbon fiber tank would be, but its a lot easier to carry weight on a belt or BC versus a heavier tank on the shoulder. I am not getting any younger LOL.
I prefere to carry tanks on my BP on my back when walking, and then its no difference.