Trying to be clearer: if we don't have the representation, server side, of the datamodel exchanged to the API, it will be horrible to:
- document it, line by line
- reuses it elsewhere, 
- tracks delta between different rather similar structure, 
- takes advantage of the compiler to check types as much as possible
- optimize the way that datastructure is produced (coupling between the data structure, part of an external contract, and internal implementation details about the logic to get it)