The Moto X Pure Edition isn’t the first Motorola device to feature a hidden LED under the front speaker grill. We’ve seen it on previous Moto X devices, as well as the Nexus 6. It doesn’t do much, since the Moto X has Active Display, but it does serve an important purpose.
When your device is too discharged to even turn the screen on when plugging in a charger, this LED turns on to let you know the device is charging. It’s definitely nice to have, even though you may never see it. Taking advantage of it as a notification LED may be possible in the future, but it won’t come without modifications.