Sometimes they are during a hard molt.Injury, exposure to the elements, insect bites and chances of infection are all reasons to not take bare backs lightly. Do the chickens seem to care? No. But if they were supposed to exist like that they would have been born that way, like the Naked Neck chickens and their bare necks. Bare backs make chickens needlessly vulnerable and good management should include keeping the birds adequately feathered in seasons other than molting.
That cushion of feathers is there to provide protection, without it...no protection. Is protection important? Apparently so or the chickens would all be bald as a cue ball all over their bodies.