Losing the glowing logo wasn't about shaving off a millimeter, but to understand why you have to bear with me as I describe LCD backlight construction... trust me, it's relevant.
For a long time, Apple used edge-lit LCDs. The backlight was a long, thin source (CCFL lamp or a string of LEDs) firing sideways into a diffuser layer. The diffuser spread the light evenly over the 2D plane.
Diffusers scatter light in all directions, so some of it exits the diffuser in the wrong direction, out the back of the display rather than forwards towards the viewer. To improve backlight efficiency, the diffuser is always backed with reflective foil.
This foil layer is where we finally come back to the logo: to light it up, Apple simply punched a hole in the foil behind the logo. The only other change required was to make the logo a translucent plastic insert.
So getting rid of it didn't save them much (if any) thickness. The real reason was that it always caused a dark spot in the middle of the screen. Most people never perceived this, but it had to go when Apple started getting serious about the color accuracy and uniformity of their displays. You can't market on near-pro-grade uniformity if you always have a dark spot screwing up the measurements.
It's even less possible to go back now that they've started using mini LED backlighting. Instead of LEDs firing into the edge of a diffuser layer, there's an array of them firing forwards. There's no good way to let some light escape out the back.