It's good practice of basic airmanship. Guys frequently add drag too early.
Airliners can achieve at least 2.5 nm (Airbus guidance) to 3 nm (demonstrated ) per 1000' of altitude. 30,000'? 75-90 nm gliding range if you fly the plane correctly.
Windmilling engines provide a significant amount of hydraulic pressure (ex. US 1549), especially if you don't move the controls.
Every descent/arrival uses the basics of energy mgt. Gliding is no different.
IMO pilots real nightmare scenario is a cabin fire.