Hello!
You are quite right, at idle power and hi-speed the prop tends to windmill creating drag, increasing the RPM you increase the drag.
It is definitely noisy and uncomfortable for the occupants, but it isn't always a problem for the engine, it depends on the engine type (piston or turbine, free turbine) and the engine can be integrated with a Negative Torque-Sensing system that increases the prop pitch to reduce the effect if necessary.
All the Turboprop planes I flew (PC 6, Be350, ATR72, Sb2000) used to slow down really well without the need of increasing the prop, but it was an option if needed and I don't remember it as a problem (within some limits).
More often was used for steep approaches or to help to stabilize the approach in turbulent and windy conditions.
Just my two cents
Evan
PS
I do downshift to help braking every day in my car...