I wouldn’t say it’s only useful for people without vanity domain names - it also adds a level anonymity should the user database get leaked (IE: it’ll be a random HME account that will look like every other, vs your specific domain name)

Also FWIW you can use it with custom domains - I use it with fastmail and have since day one (their version of it isn’t as tightly integrated as Apple but they offer this service as well). Hope this helps!