Originally Posted by
Errorname
Ultimately I don't find these reasons persuasive because having a one-note evil species is not required to actually explore any of these ideas, and their inclusion generally makes these ideas less compelling, not more.
Making non-human races diverse in culture and character makes them feel a lot more real. If your world has an evil army of Orcs menacing the nations of good, having Orcish tribes not aligned with the army or Orcs citizens in the nations of good doesn't undermine the army's villainous threat, and actually give you opportunities to develop that army by providing potential points of contrast.