Other constraints include: Maintaining cruise Mach Number, . . . as when crossing the pond on the NAT tracks, for example. So you are not at liberty to just "slow down" when you're light enough to do so, or if you can't climb.
The fact is that your airplane is not the only one in the sky and you may rarely achieve your "optimum" FMS performance parameters for the entire sector.
And even if you got lucky with extra tailwind, or less than forcast headwind, and maybe saved 1 or 2 tonnes of fuel after 13 hours, . . . when you get to the end of the rainbow early in the morning, at a place like LHR, you may be stuck in a holding pattern, you're optimum FMS descent profile for the anticipated runway is trashed, and whatever fuel savings you had coaxed out of your magic box has evaporated.
For example, in a 74, after a max gross takeoff, upon reaching FL 290, the LRC speed wouldn't be less than M.855. And if you were to "slow down" and operate below LRC, you'd be burning more fuel because of higher "deck angle" [more body drag]. Another anology might be: As when operating a boat "on the step" . . . to do that you have to be at a certain speed.