Rationale is to explain away loved-ones behavior changes. ( I would think close family would be first; Second would be extended family, and third would be close friends who are part of the extended family? I opine that a general announcement like marriage bans or really good news would be inappropriate, as it would also notify the evil scum that looks like good news for them!!
If financial predators are a concern, the LO or his durable PoA will need to start locking down all sensitive areas, especially anyone's ability to manipulate LO into changing his will or PoA. Having an official dementia diagnosis in the medical records will help prevent financial abusers from getting away with it for long, but they can still scam him out of money, take his personal possessions, cash, passwords, etc. Make sure his ATM and credit cards, blank checks, passport, driver's license, etc. are all locked away in a fireproof safe and only trusted people have the keys. All this needs to now be of concern to the person or PoA, no matter how "mild" the LO's dementia may be at this point. This abuse happens all the time, from within family and extending outward to friends, neighbors and complete strangers. You can read all about it on this forum.
One thing I have learned about this disease is that everyone who has it progresses differently, some faster, some slower and in every different way.