At what point does it become ok to have an open bed?
When the distance from the back of the truck to the front of the bed is longer than the distance from the back of the cab to the front of the truck, it turns from a Sport Utility Truck into a Pickup Truck. Typically that’s around when the bed gets big enough to haul a sheet of plywood or drywall safely.
Of course it’s OK to have an SUT instead of a pickup truck, just not as useful for construction work.