You wouldn't use this for arbitrary object serialization. Lists can only contain strings, not other lists or dictionaries.

As for stringification, JSON's data types are mismatched with pretty much everything that isn't JavaScript to some degree. If you need to serialize a 64-bit integer to JSON, you serialize it as a string because the parser on the other end is probably going to try to parse it as a double-precision floating point number. Once you've started serializing numbers as strings anyway, it's not too far to "serialize every scalar as a string".