The Bottom Line
If your cruise and descent management saves fuel vis a vis the flight plan - and mine usually did, then I can't see how the slow descent scenario can ever be better.
It is a very short sighted policy to save theoretically peanuts in descent and end up in a long approach at low level anyway.
The true saving occurs with the latest possible idle descent - even though the speed comes up over your "optimum" FMC or book speed - and into an expeditious approach.
Coming up from the south to LTN you can often get a left hand descent straight into RW 26 and if you do it right, hardly touch Stanstead airspace - gaining anything up to 450kgs and many minutes a shot. Follow your FMC routing and descent on this one and you lose - every time.
The fact that flying slow cocks up your colleagues - possibly from your own airline, makes this individually calculated policy even more daft.
Bottom line, pilots have more flight experience than FMC computer designers - use it!
FC.