I recently re-watched Apollo 13 (although it was sort of indirectly; I watched it with the commentaries by director Ron Howard and Jim and Marilyn Lovell), and I noticed some significance in the composition of this shot:
In his commentary, Ron Howard briefly mentions that this shot was achieved with a diopter, so that Marilyn in the background and the NASA squawk box in the foreground are both in focus. This sort of visual presentation highlights the distance between the two, which in turn emphasizes the space between Marilyn and Jim. In this shot, she's far away even from the object that merely represents him. At this point in the movie, the Apollo 13 spacecraft is going around the dark side of the moon and is therefore out of radio contact, and these circumstances also contribute to this sense of separation.
