Let's say you transmit an object of type Person, that is also a Student and a MartialArtist. Your inheritance graph may define that a Student is also a Person.

Let's say you transmit an object of type Person, that is also a Student and a MartialArtist. Your inheritance graph may define that a Student is also a Person. So not sending the Person type could be fine. But would you define a common subtype for Student+MartialArtist, just because your data serialization handles only one single type per object? Obviously no! You want to send your object with types "Student" and "MartialArtist". I.e multiple types.