I think there is a subtle difference between using the flaps as a last resort to maintain profile just within speeds and a gradual continuous descent with smooth deceleration and configuration changes, well below the limitations to hit a loc/glide intercept exactly on slope, five flaps and idle power.
ie 230 kts clean will give you plenty of penetration until you know you wont drift high. Then select clean speed and dirty up avoiding the amber. I would always use the speed brake/vs if atc/proc required slower and there was any thermal activity, due to the a/p commanding 500feet rod to facilitate slow down, when on average you need 1100/1300 in this sort of phase to maintain 3 degrees. I prefer this to level flight especially if you get more vectors than you need , continuous can be adjusted to suit, levelling you may just be dragging things around the sky. Level flight is a bit like fixed card adf work , parallelling the bearing, methodical:continous done properly is more like an rmi , fluid.
Dodgy analogy I know
Right thats as clear as mud then.
PS i know the flaps on th 737 dont contribute much drag normally as has been mentioned , but its more than you need on a wide vector if you loose an engine.