Shrike, obviously if your airline has an aircraft planned to fly all day,ie if you get late the last flight of the day is cancelled then yes flying higher speeds makes commercial reason,,,but passengers im sure dont quite enjoy idle descent from high level at 320 kts,quite a descent rate..and fuel burn is highly increased..
If you are early however i decrease the ci to save fuel as a mere 100kgs saved/aircraft/leg in a large fleet makes quite a saving at the end of the year
