Extra fields can't have formatters, formatter settings or a formatter summary. field.module and field_ui.module could provide this, so that other modules (that define extra fields) can use those settings.
A use case:
A module that provides registrations on nodes also provides a link to the register page. That link is an extra field. It would be excellent if the developer could enter a link text (to be used by the registration module in MODULE_view() eg). Another module could then create another formatter for that link to make it open in an overlay (and then alter the entity display).
A few issues:
* the field type
* the formatter name
* where to store:
** which formatter is used
** its settings
And that's just what I came up with.
field and field_ui don't even have to do all the work, but field_ui would have to 'open up'field_ui_display_overview_form() to do the formatter thing for extra fields too.