I feel very comfortable with the argumentation brought by Rick and the others.
< "I am thinking more of extreme low light photography…" >
Exactly. This is where the collected colour information comes in the play. In low key condition, the D800E will see very faint colour shifts because it is not colour blind. A somewhat dark brown beside a somewhat dark blue or green will be recorded as such. But in the dark, all the cats are grey! A monochrome rendition of the D800EM would only see a dark area beside another dark area…
In the times of B&W films, a better film had simply an emulsion that was offering more shades of grey: a greater latitude when capturing colour values and recording them in monochrome shades. That were millions of colours regrouped in 11 greys!
Then came the chromogenic emulsions, bragging to perform on a 31 shades of grey base, plus black, and plus white. Wow! Three times more! That were millions of colours regrouped in 31 shades greys! Now, in the digital era, we are talking about a minimum of 256 shades of grey! This is still not much beside the XX millions of recordable colours!
If you record all the colour information, you may, using the six sliders in the tool box, tweak the way colours are shifted to or from a given shade (or level) of grey to another.
Even with my beloved Beseler 4x5 colour enlarger and working with XP1/C41, these performances were beyond my reach.