The first three things I learned in scn were:
1) You are a spiritual being, therefore
2) You are immortal, and therefore
3) You have lived before.
I think these points were pushed before anything else because they are ideas that many people find it easy to agree with. Reality is agreement, and Hubbard found some things people could easily agree with to get them on board with his reality.
At the mission where I got started, all the scn books were sitting out in reception where anyone could pick them up and browse, and we did. (They weren't shrink-wrapped as I hear they are now.) So I looked into "Have You Lived Before This Life" and "History of Man" before I even did the Comm Course.
I think we were to a degree pre-selected for believing in at least the likelihood of past lives. If you couldn't accept the idea that you might have lived before, I don't think you'd have hung around long in an environment where that sort of thing was taken for granted.
Later, on the HSDC, I read that people who couldn't run past lives "seldom reach basic on any chain" and therefore need a past life remedy. One remedy was to list "What (attitudes/emotions/sensations/pains) would make one unwilling to look at past lives?" and run the reading items R3R (run them as Dn chains).
I never knew anyone who needed a past life remedy. Everyone I audited went whole track easily. I ran lots of Dns as an auditor before I ran any as a pc, and when my turn came I went whole track easily too.
I now think the reason we all ran track so easily is that what we were really doing was just making stuff up, though I certainly never admitted this to myself at the time. I knew there was the possibility at least some of it wasn't real, but I didn't worry about that. Hubbard says (in DMSMH?) that if the pc is running dub-in it will eventually run out and he'll start running the real stuff.
I think what actually happens is that the dub-in (fantasy) you're running begins to SEEM more real the more of it you run. The dub-in is accepted by the auditor, who mustn't "invalidate or correct the pc's data," and this has the effect of tacitly validating it as real.
I think Hubbard knew what he was doing here. He wanted pc's to indulge in hundreds of hours of fantasy because he knew it would eventually impair their ability to distinguish fantasy and reality and make them more willing to accept whatever he said, no matter how bizarre, the Xenu story being the ne plus ultra. He said that "engram running gives the most case gain," and from the late 60's through the 70's he kept increasing the amount of Dns you needed to run to get up the bridge.
So yes, I think past lives are an essential part of scn. If you call yourself a scngst but don't believe in past lives, then I think you're redefining scn in a way Hubbard wouldn't have approved of at all. Past lives were an essential part of his scheme to control us.