Reality is they usually replace whole panels if there is any significant damage (steel or aluminum), so not a big deal from a repair point of view.
If someone dumps a palette of bricks from 5' up into my steel truck bed, I'm going to be ticked off. 99% will never see that kind of use. Most construction guys if they use the truck bed hard will put a bed liner/mat or a piece of plywood to take the brunt of the hard abuse.
On the other hand, try to find a 8yr old truck in the midwest where the wheel wells aren't rusted out. Aluminum will eliminate this issue.
PS: Fiberglass in common usage reference GRP (Glass Reinforced Plastic). Carbon Fiber is a specialty product used to replace the glass fibers for more strength and in common usage is often still refereed to as Fiberglass. Honestly, it's surprising they don't use more GRP. Love em or hate em, look at the Pontiac Fiero or the Corvette. 30-40yrs old and the bodies still are perfect if they haven't gotten into a crash.