DavidReidUK "The flattest glide (ie minimum height loss per horizontal distance travelled) corresponds to the bottom of the drag curve (max L/D).
Fly any slower or faster and you will be draggier with a resulting steeper descent. "
Quite correct.
But what is the effect on the drag curve of having one prop still spinning and one feathered? Its like flying a glider with the airbrakes stuck fully open on one wing only. (I have been in that situation and it isn't nice). The speed for best L/D is considerably reduced, and is close to what would have been minimum sink speed, at the same time stall speed is increased.
Additional speed is also required for safe manouvering and for a controlled flair.