In the DocumentType Editor a document field doesn't have an consistent logical name except for its low-level JCR Item name or its caption which should be localizable

The consequence of not having a consistent logical field name forces a developer to always use to the explicit namespaced JCR item name to refer to a field, like in code using the new ContentTypeService.
While a logical name might/should not be required, having it optionally available (and then unique within a Document type, including its parents) makes it very useful and possible to use such a logical name independent and abstracted away from its underlying (and then changeable) JCR item name.