4Dogs
Read my second post carefully. I didn't invent this Boeing advice - it is there in black and bloody white and I quoted from the B737-200 FCTM Date August 20th 1976 - Page 04-60-02, first paragraph, line 5 which states:
"Normally, extend the flaps on the flap/speed schedule IN LEVEL FLIGHT" (my capitals).
Succeeding Boeing FCTM's have been apparently been the subject of significant legal re-editing in order to minimise the risk of litigation eagles seizing upon the slightest excuse to make a killing. There was far more amplification of Boeing intent in the early FCTM than you see now. The responsibility of how Boeing advice is interpreted is now left to individual operators - with the resultant wide ranging and often questionable points of view.
I certainly do not recall stating (as you have interpreted) that Boeing do not permit flap extension in other than level flight. Where on earth did you get that from?
Also I fail to see the correlation between extending flaps at the correct speed in level flight and in passenger and cabin staff comfort if the flaps are extended in descent.