Nick,
I see your point. Indeed most jetliners and business jets have about 20 to 1 L/D.
Yes, that makes jets pretty efficient at their design speed where speed at any cost is the goal. For personal craft where speed is not important, the goal is to fly with low cost, low power engines.
Actually, to be precise, my glider needs a low minimum sink rate in a thermal. L/D is not a factor in a thermal. So for a helicopter, it might be better for Dave to look at minimum sink rather than L/D. Personal aircraft that fly with low power usually have a low sink rate as well.
Minimum sink speed is just a bit above stall speed for a glider.