Calling api/Rest/v1/Users/{id} requires that you provide an email address along with other details. The admin interface does not require providing users with an email address. This means updating a user with the API may require "faking" their email address which pollutes the data.
All parameters should be optional for this API call and the call should only modify the fields which are provided. I should not be required to modify a user's department/username/first name/last name/email address when all I want to update is their status or phone number.
I second this. The only thing that should be required on an update call should be the unique identifier for the individual. Aside from needing to fake data, requiring the other fields invites issues with data integrity and allows for more human error, especially when multiple groups have access to the same set of individuals based on their unique identifier.