Some missions need a faster PC than other missions.
People with low-end systems would have to adjust their settings all the time.
Or they just sacrifice the potential image quality in the less complex (perfomance-wise) missions.
And one other thing would be possible with this feature:
BOE style missions. Everything will be a bit less beautiful, but the performance sill be fine.
(The slower the PC, the less image quality you get.)
This means the modder teams could create super-high-res maps and high-poly ships, as long as the create lods. In a few years´, when the PC are faster, those maps and models won't be a problem for the hardware anymore.
This means the mod would evolve along with the PC hardware.
(Well, at least that's possible

)