Reasons stated taken, now reasons to have a personal domain:
- not being bound to a specific third party :: even if you do not host your mails with your domain you can transfer it to another vendor, without changing addresses, aliases etc so transparently for your correspondent;
- mails and website, a single domain :: easy to reach you since typically mails are meant to communicate not to be secret tools for a selected cohort of friends;
- since the domain is like a postal address, something a Citizen should have to be, well a Netizen need it as well, and it's the basic brick to form own digital life in a connected society, domain, mail, personal website, perhaps homeserver are the tools of the trade like a home, a car, dresses and so on are for the physical world.
When I'll be dead those who eventually snoop my mails (that are certainly already snooped by gazzilion of third party at least in automatic ways for generic data mining, antivirus scan and so on) I'm pretty sure I'll not care... If I'll have heirs they'll be already instructed on what to do as for anything else.