When you coast with engaged clutch, you will engine brake. Energy doesn't go to the engine for free. Adding the fuel to counter the engine braking is less efficient than letting the engine run idle while coasting due to losses in the power-train. Plus, more efficient cars still have to inject fuel. The electro-hydraulically controlled conventional gearbox of the Lupo 3L disengages the dry clutch while coasting for a reason.
Of course, engine braking is obviously more efficient than friction braking, but that's not coasting, that's engine braking.
Another funny note is that "conventional" automatic gearboxes have the highest gear (usually the only overdrive gear) have a coasting clutch on it, so it's not even technically possible to coast with engine engaged. You're always idling if you let go of the speeder, losing energy to the torque converter that doubles as oil pump for the transmission.
Of course, engine braking is obviously more efficient than friction braking, but that's not coasting, that's engine braking.
Another funny note is that "conventional" automatic gearboxes have the highest gear (usually the only overdrive gear) have a coasting clutch on it, so it's not even technically possible to coast with engine engaged. You're always idling if you let go of the speeder, losing energy to the torque converter that doubles as oil pump for the transmission.