DHD
One of the benefits of using a fixed de-rate is being able to lift a greater weight off very short runways when you are 'relatively' light.
In a -500 TriStar for example we could lift up to 20T more off short runways because we were derating to a 22B thrust instead of the full 524 power. This amounted to some 8000lbs less per engine. Admittedly the short fuselage -500 was quite Vmcg limited, but even so I am surprised that full power is available when using a fixed de-rate.
This is not the case on the 'bus'.