One things that all TD's will agree about is that safety is a huge priority. One element of running a safe theatre is making sure that the theatre is safe when it is dark, and everyone (or most everyone) has gone home for the night. I am not sure (but would be interested in hearing) about where the origin of the ghost light came from. Regardless, making sure that the theatre space has light is a good idea. Between changes in the space from one show to the next and the assorted dangers of a set (lefts, open pits, ...) a little light wont hurt. Especially when the person coming onstage could be unfamiliar with the current set up. I have also seen about as many types of ghost lights as I have seen theatres. One of my favorites though is the Wobble Light
http://www.wobblelight.com/ as in weebles wobble but they don't fall down.... Durable, fun, and safe. Those aren't words you can always combine so easily.