In light of the work being done on the unified Entity Field API, is Field API's concept of 'extra fields' still relevant?
I don't think I was around Drupal when this was introduced, but it seems to me that it only serves the purpose off adding 'stuff' to entity displays and forms that are *not* configurable fields. Given that we now have a new API for entity fields (formerly known as properties), and we want to move in a direction that allows existing field formatters and widget to be used by them, I think that a lot of special-casing code can be dropped and cleaned up.