Flap use
Never heard or read that Boeing would recommend extending flaps while in level flight alone. Flew the 727, 737, 747 and DC9.
What I think Mr B meas by " The flaps are not normally used for increasing the rate of descent". is that flaps should normally used as speed brakes in the initial approach segment (Read STARS) for the following reasons:
Expected speed in many STARS are much higher that the usual speeds to configure aircraft with the first segment of flaps. One has the use of Speedbrake/Spoiler or in extreme cases landing gear.
Using flaps as speedbrakes is not a good practice as it increase wear & tear of all flap components not least the jackscrews and drivemotors. The possible reconfiguration of aircraft due to ATC speed requirements is time consuming (1-5 with B747 takes it's sweet time and vice versa). Is selecting flaps back and forth a good idea?
But if you are heading down the glide, speeds aremoslty quite low and well within flap speed range.
These profiles are guidelines and should not prevent PF using his/hers own judgement on what is best for any given situation. Also many airlines have procedures such as lowdrag approaches with late configuration of aircraft (Saving fuel) that differ from those of Boeing.
Hope this made some sense
AD