One of my favorite lines in "Is There in Truth No Beauty?": Miranda tells Kirk (in the rose garden, I think) "I've never been to Earth." I like Diana Muldaur's way with this line. The Making of Star Trek (1968) includes memos that discuss why the series would never return to Earth of the present. Good reasons, as I recall. One of the selling points of TMP was that fans would finally get to see Earth, or at least Star Fleet headquarters. But that movie made it easier for later movies (in 1986, 1996, 2002, and 2009) to return to the Earth-under-threat scenario. Five such movies are already too many.