Encounter.diagnosis currently requires a reference to either a Condition or Procedure.  However, in many cases, the diagnosis (or diagnoses) on an encounter are just lists of codes (ICD-10, SNOMED, etc.) entered by clinical/billing staff and not actually separate entities in the chart which could be represented as a full resource on their own.  For example, the admitting diagnosis may be simply W56.22 (Struck by Orca), and the patient might end up with a several Condition resources describing the results of that (broken bones, concussion, etc.) and the Procedure resources that are performed in the process of treatment.

It seems like Condition.code (which is a CodeableConcept referring to value set Condition/Problem/Diagnosis Codes) is a more appropriate representation for this kind of data, and the Conditions & Procedures documented on an encounter should be separate elements from any of the diagnoses (admitting, primary, working, discharge, etc.).