It doesn’t have exactly the same effect: you get lower contrast. When you reduce the brightness of the backlight, blacks and whites alike become less bright. When you reduce the RGB intensities, the whites become less bright but the blacks stay just as bright. How big a deal this is depends on the technique. On OLED panels there’s approximately no difference, but it’s a huge deal in low-light scenarios with most LCD panels.
I just tried it on my MacBook Pro in a perfectly dark bathroom to double-check.
Yes... at minimum brightness #000 isn't pure black, it's a very, very, very dark gray.
But nevertheless -- there's still plenty of contrast. So while it's technically true you get slightly lower contrast... I don't see how, in practice, this would be a problem for anyone who's simply doing reading/writing/coding.
(Remember, many coding themes are intentionally lower-contrast anyways -- black text on white is really dark gray, bright text on dark background is a dark gray background. So it's doubly not a problem.)
