Yes, that might be nice to have. The One Laptop Per Child laptop (XO) had a dual-mode display with a "Reflective (backlight off) monochrome mode for low-power use in sunlight" [1]. Does not seem to have caught on though. Wasn't the same, but as far as I remember, it was alright.

I guess the majority of users would not want a greyscale display (or at least manufacturers think so).

[1] https://en.wikipedia.org/wiki/OLPC_XO