When we had pickups, we never ran covers on them. Did have bed liners, and sometimes either a commercial or homemade toolbox up front behind the cab. Except for a small amount of tools, the pickups were just used for temporary transportation. Those were work trucks, though.
I do have friends that have covers on their trucks. Not work trucks, just basic transportation, and mostly I think it's for looks, with one exception. One friend used to carry large RC aircraft in the back of his truck, with a hard cover. That worked great for him. If I remember correctly it folded over to the side with structs, but not positive.