Null is in there because it exists in the real world (unfortunately). There's no putting the genie back in the bottle (although there's an admonition against using it in the spec). Pretending it doesn't exist would cause more trouble in the real world than letting it be.

The string-type prefixing is there so that you don't actually need a schema for 80% of use cases (schemaless data is usually enough). A schema can be added later if desired, and I've been thinking of what that might look like, but it's at a different level than the encoding format and can be developed independently.

The primary purpose of this format is to serve the 80% use case (general purpose, ad-hoc, hierarchical, schemaless data with fundamental type support - or something that easily fits within this paradigm). For the remaining more complicated 20%, there are custom types and at some point in the future, schemas.