I don't believe in reincarnation or soul Disks, but I'm interested in the extent to which such claims can be rendered logically coherent and consistent with other well established data about the afterlife.  I'd like to offer 4 lines of questioning:

(1) The OP's original question is potentially a fruitful one.  Why can't the whole soul Disk incarnate in one incarnation?  If not, why not?  If so,  what might be gained that the incarnation of a soul fragment could not achieve? 

[[Because there would be too much energy.  My higher self/over soul once enabled me to experience what it is like to be an over soul like being. I was aware of numerous incarnations at the same time. It wasn't difficult to be aware in such a way because my vastness handled all of the information quite easily, and it felt completely natural to experience in such a way. There wasn't anything negative about it. Interesting thing was, I still felt like me.  There is no way all of that energy could be fit into a body.

I stated this elsewhere, when NDEr PMH Atwater experienced her Soul, she understood that it was way too big to inhabit a physical body. She understands that she is just one finger of her soul. Other information I received made the point that there is no way the entirety of a soul could be placed in a physical body. As I have grown spiritually I have found that amount of spirit energy I feel can be overwhelming at times. For example, I can feel it streaming out of my face.

Considering that people are often very slow to learn and change, it is understandable that a soul would need more than one incarnation to learn what it needs to learn. It  seems as if one of the things my soul needed to learn during this incarnation, is to not be afraid of unfriendly spirits. It took a number of years to improve my wisdom about this. One thing that helped me was what previous Disk (soul group?) members learned. I don't mean to brag, I've had a kind heart and loved God since I was a young child. I was never mean to other children. In fact, I stood  up for kids that were bullied. How did I get to be this way? Don said elsewhere that souls could learn before they incarnate in this world. I agree. This world is just one way of learning. But I also believe that the lessons of more than one soul incarnation can be valuable.]]
(2) But  questions in turn are related to a more basic question: How is it even meaningful to posit postmortem life for a total soul disk?  Suppose more and more soul fragments are "retrieved."  What difference would that make to the Disk Self as a whole and why?  And how would such differences ever be discovered?
[[Going by what I understand, fragments aren't disintegrated after they rejoin a Disk. Perhaps this is significant: I don't recall either Robert Monroe or Bruce Moen using the term higher self or over soul. They used the terms I-there and Disk. I wonder if they experienced something like a group soul/soul group. If higher selves have multiple incarnations, what are the margins between these selves and the selves of other Higher selves that share the same soul group? I don't believe the margins are significant. I figure that members of a soul group are able to share information with each other completely. This is a very effective way to learn. It makes it so none of us on and individual self basis needs to incarnate repeatedly. Sometimes past life memories might come from a soul group member.

Robert Monroe wrote of a new self being created. In his NDE account  Thomas Sawyer spoke of something similar when he spoke of reincarnation. In his NDE account, Ron Krueger spoke of meeting his previous selves, and they were all joined together in a cluster. I believe that in truth we are all connected, and not separate form each other.

I'll have to continue my response later, because I have to take care of something now.]]

(3) Swedenborg claims that his apparent retrieval of past life memories was shown by guides to be a misunderstanding based on his lack of awareness of unconscious soul mergers in which the memories  of undetected spirits flow in and appear to be one's own from a prior incarnation.   ES claims to have learned this correction by direct astral experience.  How can such a discrepancy be explained or reconciled with the soul Disk view? 

[[There is a book called "Thirty Years Amongst the Dead." For 30 years Carl Wickland  and his wife helped people that were possessed by Earth bound spirits. Through one case they found that some people are possessed by people that are trying to reincarnate, because such spirits believe that this is what you are supposed to do after death.

Going by the information I have received, what some sources say, and what makes sense to me, reincarnation doesn't happen in the conventional way. It is more of a Disk-like activity that might extend to soul groups. Past life memories can come from souls people merged with before they incarnated in this world. Some of these souls could be Group Soul spirits.

Perhaps some of those Ian Stevenson cases were possession cases.

I'm weary of accepting what hypnotists have found through past life regressions. Hypnosis doesn't seem like foolproof method of obtaining information.]]

(4) Also, consider evidence from ADCs, NDEs, channeling, and the Bible documenting a post-mortem career for new arrivals.  How are such careers" possible if the hypothesized soul Disk is focused on new incarnations?  What evidence is there for an interruption of a postmortem "career" for a new incarnation?