"Heck if Mr. or Mrs. Boeing did'nt want us to use Flaps 10, then why put a detent there."
I'd always thought it was left over from B732 which had a fixed speed schedule of 210/190/180/170/160/150 for up/F1/F2/F5/F10/F15. In USA I believe ATC liked to use these speeds for 'pushing tin' and crating & maintaining correct spacing. I believe 160kts was a common speed on approach, as indeed it is at many UK airports.