Maybe Not So Rare
Plurality is not rare.
Prevalence rates of DID alone within the general population, which is not the sum total of all experience of plurality, are 1-3%. That's about as relevant as bipolar, and more so than schizophrenia. Some people who are schizophrenic may consider themselves plural, there's the other dissociative phenomenon that require a sense of plurality — P-DID where there is passive influence from background entities, or OSDD where the plural group entity experiences little to no amnesia thus doesn't qualify for a DID diagnosis but still are a plural entity and experiences distress from C-PTSD symptoms.
The so-called "rarity" of these disorders is not how prevalent it is, but how often they are accurately diagnosed. In other words, their rarity is not the burden of the person seeking a diagnosis, but the blindness of the professional industry that fails to recognize our distressed brethren when they're sitting right in front of them in their office, asking for help. We will give a passing nod to inaccurate depictions in the media, and covert systems who are not yet comfortable enough to reveal themselves in a therapeutic environment. The former because professionals may be mistaken as to how plurality presents itself in a clinical setting, and the latter because frankly some plural systems are extraordinarily good at "passing" as singular; it's part of their defense mechanisms.
But it's there. And in a clinical setting, it's there more often than professionals think.
If DID is 1-3% of the general population, and say 30% of the general population seeks treatment for mental health issues, then as much as 3-10% of the customers walking into a clinical setting could have DID — again not covering related issues. That is to say it is likely that more than 1 out of 30 to 1 out of 10 people walking into the office may have an undiagnosed dissociative disorder.
Sorry, professionals — that's not rare.
This isn't counting sub-clinical presentations of plurality. Normal folks who get along as a group entity and maybe haven't even realized that is what they're doing; they haven't leveraged it, or explored it because our society doesn't encourage leveraging or exploring it. A lack of role-models, of forums for exploration, or encouragement to seek out one's plurality could very well leave wellsprings of humanity's talents untapped.
So, the next time someone walks into your office and tells you there's a lot of "people" in their head and you feel the auto-rehearsed retort of "But that's so rare…" coming to your lips — stop. You're being invited into a new world you have not experienced. Listen, don't talk.