I am pretty sure I know the reason, and it makes perfect sense.
The "light" on the back of older Macbooks is actually just a hole where the screen's back-light shines through. The problem with this is light from behind the screen can also shine through the other way. I have experienced this countless times with my MBA; the sun shines through my lid and makes an Apple-shaped light shine through my screen.
The new design means light cannot shine through and make that annoying Apple shape on your screen. It was pretty bad on my MBA. Not so much on a MBP, although still present.
EDIT: If you want to really see this, turn your screen brightness down and hold your MBA/MBP up to the sun whilst looking at the screen. That's when it's most apparent, although there are other times it can get in the way too.