From what I have read, they were intended to be lit but the lamps were left out for cost/problem reasons. I think that they are in all of the TFTC back boxes. I know that they have been in the two that I have worked with.

I am sure that you could tie sockets in to the backbox GI and illuminate them. LEDs would solve any heat/power issues that may have kept them out of the production machines.

I think that there were a lot more of them with these than people think. I have worked with a couple of different ones and both had the inserts. I doubt that I was lucky enough to stumble across two engineering samples.