That "formula" allows the service to identify you the user because it's your domain (or group you into a subset of users if you share the domain with other people). Apple's relay service right now gives the application zero new information about you.
Again, from a customer angle I’m not 100% sold on this. I don’t want to miss an email because Apple servers are down, and if Apple decides to kill an app I don’t like, I also want the ability to disagree. Separately, I want something that wouldn’t be a pain to use on non-Apple devices.