The slowing isn't using gas - its not like they put the plane in reverse to slow it down. They're descending and trading potential energy for energy to maneuver at low altitudes. The thrust is near idle.
True, but not every approach to an airport is a nice constant vector.
Continuous descent operations are the optimal, but there's plenty of suboptimal airport approaches that require level flight at a height where fuel efficiency is far less than at cruising altitude.
Do you have any statistics to say it happens so often that airplane landings are so suboptimal that they use anywhere near climbing fuel rates and times that it matters to the overall discussion? It seems unlikely to me.
Continuous descent operations are the optimal, but there's plenty of suboptimal airport approaches that require level flight at a height where fuel efficiency is far less than at cruising altitude.