The reason we deal with Greeks by themselves or Romans by themselves is that they were distinct civilizations for centuries. But in the Middle Ages, political units like France, England, etc., were part of a common civilization, so it makes sense to talk about them as a group. The only time this might get confusing as far as I can tell is when you’re trying to learn about the particular political history of a region. The broader cultural and religious features are shared for the most part.
