Is there some reason they shouldn't just use 100% thrust at takeoff and make sure the cargo being carried was less than the maximum capacity?
These engines are optimized for certain turbine speeds. By staying within the recommended ranges they reduce wear & tear and improve efficiency. In addition, max acceleration is harder on the passengers. Think of it like driving a car. Do you accelerate from every stop by pressing the gas pedal all the way to the floor or do you match how much gas you give it to the driving conditions and who and how many people are in your car?