Thoughts:
1). If they have notably different personalities, they would find troubles in life because other people would likely find them unpredictable, mercurial and off-putting. If they failed to share memories, they'd be like people with traumatic head injuries - you'd have to keep telling them the same things (perhaps for each personality) and that gets vastly tiresome.
2). How do they get these personalities? If it is by lived experience, without shared memory, let's say you had 4 personalities, to get to the maturity of an 18 year old might take 72 years as each personality has to go through that. Very not functional.
If it is by some form of loading (if personality can be digitized and restored onto different wetware with some good transfer software... we do it on computers so it may one day work on brains), then an 18 year old could have multiple personalities. For that matter, some of them could be considerably older and more experienced than an 18 year old body. That could be awkward for one of them who is effectively 45 being treated as the visually 18 year old they are physically.
4. If they don't share memory and have triggers, that sounds more like a classic multiple-personality illness. (Some share memory, but some aren't aware of one another). That's often a result of vast trauma. Does that mean all these MCCs would effectively be trauma victims? That's a rough life.
5. If you can load personas and they can be hidden with undetectable (at least on the surface triggers), you've just figured out how spies and assassins would operate. Take someone usefully placed, pay a noctural trip, a little meds, load the assassin persona, set triggers, the person wakes up with a headache and has no idea they've been turned into an unaware assassin. Also means you can get close to key targets and kill them and it might take the authorities a long while to discover the root cause.
How to generate, create, and play such characters would depend a lot on:
a) shared memory or not
b) whether age differentiated personas could be loaded or somehow develop
c) triggers
d) how functional you expect them to be in the game (random triggers, no shared memories, unknown characters that the others don't know about, etc. would tend to make you interesting from a fiction perspective but not very playable in most groups)
e) how maturation would work for them
f) how careers could work for them (if you flip personas multiple times a day, I see odds of re-enlistments lesser, more likelihood to have problems, etc)
I've had the experience of having several abuse victims as very close friends. They have very rough PTSD (one of my friends has survived when his doctors said most would not and the other has tried multiple times to do self harm). Intense PTSD isn't multiple-personality, but it has elements - in dissociative periods, they function as a different person (one of my friends dissociated in Ottawa and found himself arriving at Montreal having driven there and he had no memory of getting in the car, why or the like... in other cases, he's interacted with people while dissociated because he doesn't know when the spells might come on... and he has no idea how he interacted with them or what he said to them... even in some cases if he interacted with them). It's terrifying to him because he has no memory of these times and no control over what he may have done or said (as his normal persona). Most of the time the episodes are an hour to six or eight hours at the outside, but once or twice they were about a day. He's apparently functional (people who he talked to after that he had talked to while he was dissociated didn't seem to realize he'd been dissociated). Most of the time he just stays housebound. But imagine if he drove somewhere and had an accident dissociated then couldn't testify in court or even document the accident for the insurance company later....
My other friend had suffered even more horrific childhood abuse. That led to modern memory being interspersed with old memories to build new traumatic memories. It isn't always full fledged flashbacks, but instead small elements. She has had problems having any sorts of friends or relationships because some trigger or just the damage she has to live with will pull elements or bits of memory from the original traumas out but fit them into the current event and a new memory is written into memory that is NOT accurate and yet the sufferer totally feels it is and they can't do anything but act on it... losing friends and loved ones. For them, the memory is real and feels accurate. Also, they have been disbelieved by so many people (and have turfed so many people from their lives) that the victim cannot bring themself to admit 'I could be totally wrong and my memory is entirely not believable and I can't distinguish truth from false memory'. That's just too much of a jump, so the brain clings with brutal force and reacts with blazing anger at any attempt to sew doubt or try to suggest their understanding has a few respects in which it does not align with reality. And trying to sew that doubt... bang... and that person is evicted from their life, even if that leaves them alone (which is also terrifying). They jump from person to person and use them as a lifeboat for a time, then jump again due to new trauma.
So, I think the 'don't share memory' reminds me a lot of dissociation. I think people would be terrified. Similarly that would be a part of unknown personas. That's pretty horrific too from my first friend's experiences. I don't see that sort of setup as being workable. And all that terror, plus the original trauma, means comorbid conditions like anxiety/panic attacks, not being able to go out of the home, bipolar or narcissistic behaviours, and the list goes on.
Now, if you had shared memory, ability to mature all 4 personalities concurrently or at least in a very slightly more time or to load matured personas, you could switch in and out between the great administrator, the great scientist, the great mathematician and the great poet as needed. Or maybe you could even do some form of gestalt personality by accessing more than one at once. That would be really advantageous, but that might be a game breaker if these are folk that are in the same millieu as regular folk.
Then again, maybe they'd all get hit with bad ADD/ADHD because the gestalt personalities would bore easily. I've seen what a bad case of ADHD can do to someone over the long run - brilliant work if it can be done before the next pretty shiny thing, otherwise no completion... addictions looking for new stimulation. Myself, I'm glad my intelligence was only ever about 1 std dev beyond average (maybe 1.5). My friend with about 3 std deviations has had a rough life because of how very easily and quickly he bores and has to seek other stimulations. I think it it leaves great art, but it surely leaves day to day life a shambles.
Anyway, too many ways to slice this. But there's more problems that could apply than one might first optimistically consider. Just to throw one more out: Personality A wants to marry another person. The other personalities vary from being ambivalent to the other person to outright hating them. That's a really awful situation to try to imagine a good result from.
TomB