In the case of an engine failure or shut down where the propeller is free to turn ie not thrown big chunks of metal out the back, the propeller is driven by the airflow passing through it. This causes a big increase in drag which would reduce excess power available and reduce control authority due to more rudder needed to oppose the yawing moment caused by the drag in the multi engine case and a reduced glide range in the single engine example.
If the engine fails and the propeller cannot rotate, but is not feathered you still have extra drag from the blades being flat on to the airflow. Feathering reduces the drag and should stop the blades turning. Composite blades are a little beyond the required knowledge in ATPL ground school from memory, I would imagine even with the curved blades the reduction in drag is worth feathering the prop.