bah, when will people learn.

yes, its a bad habit.

it uses MORE fuel than leaving your car in gear.

people have this misconception that higher revs = more fuel. its just not correct - the load is what determines the amount of fuel going in. when in gear and decelerating, the injectors shut off as the momentum of the car keeps the engine turning whilst the clutch is engaged.

when the clutch is out, or the gearbox is in neutral, the car has to be kept at idle to avoid stalling, which obviously uses fuel.

as others have said, it also means you have less control. what about if you needed to accelerate suddenly?