There would never be a reason to use windmilling props in flight for electrical generation. Even when descending, airliners like the Dash 8 need at least some forward thrust in order to maintain control and stay on the glide slope. They only use reverse thrust for a few seconds during the landing roll.

Note that these are more like turboprop quirks, not "airliners". Turbofans will idle happily from cruise to final approach, Dash 8 will not.