I assume you are creating a language that humans are going to type by hand. For humans it is easier to write in a mixed content form, because it matches the way the brain works. Create a simple schema language for simple structures. This makes it harder for humans to create documents. Create a complex schema language for complex structures.
Humans can create documents the way they want.
So you don't want to have complex schema, or schema writing is only for the highly intelligent people. It lowers productivity and creativity when writing programs for complex datafiles. Maybe humans don't like it, but forcing them to write structured documents, forces them to think more about what they really want to express. Loosely typed and natural language like expression are more likely to be mis-interpreted. However, I've always thought about data files to be computer generated.