Well,

here's a good handling of at least one of Hubbard's BT's:
"TONJA AND THE BUMP
Tonja Burden's recounting of her time with Hubbard includes a story about how Ron was developing
this fairly sizable bump on his forehead. This bump is visible in one of the photos taken by Jim Dincalcy in New York in 1973.
She was in her mid-teens by this time, a true believer, and that bump bothered her.
She had observed many negative things about him, but these did not bother her the way this bump did. She could somehow explain away these other things without making her feel doubtful that he was the god that the entire ship considered him to be. But this bump....
One of the things that bothered her about the bump was that it obviously bothered Hubbard. He used to wear hats and would carefully position them, while examining himself in the mirror, so as to cover up the bump.
It would occur to her that if it bothered him that much, and since he was an 0T, why didn't he just make it go away!
It finally did "go away," being surgically removed by Kima Douglas, his "medical officer," who opened it up and cut out the fatty deposits."
From online scanned version of Bent Corydon's book, "L. Ron Hubbard, Messiah or Madman?", page 374.
http://www.xenu.net/archive/books/mom/Messiah_or_Madman.txt
BT's as created by Ron are not ghosts or angels or demons or elementals or pixies or sprites or powers or principalities, (everything, including perceived beings from human religious belief systems were an "implant" in the Scientology cosmology, remember?) they are dead
space aliens of a distinctive species (Marcabs) who are either so unconscious they are "asleep" or are so horribly introverted that all they can do is cluster and cling together and hang on, trying to be your liver, or some other body part... (hence Body Thetan)
It's not a metaphor for something else. The man was a Science Fiction and Fantasy writer...he lived in his head most of the time (like most writers do), and made stuff up for a living. He made all this "Space Opera" stuff up to sell his "processing" to people.
Sadly, I do think that he was greatly influenced by some varying levels of mental illness throughout his lifetime, and the whole BT creation to me seems to be a product of this:
http://en.wikipedia.org/wiki/Delusional_parasitosis
"Delusional parasitosis, also known as Ekbom's syndrome, is a form of psychosis whose victims acquire a strong delusional belief that they are infested with parasites, whereas in reality no such parasites are present."
