I am writing a simple REST API in Python using [Flask-RESTful](https://github.com/flask-restful/flask-restful/) and their [documentation says](https://flask-restful.readthedocs.io/en/0.3.6/reqparse.html) that they are planning to deprecate their object serialization (reqparse) in favor of serializers like [marshmallow](https://github.com/marshmallow-code/marshmallow/)
My API is reading and writing from a MongoDB doc store, using [Flask MongoEngine](https://github.com/mongoengine/flask-mongoengine).

I would very much appreciate an example of a use case where I would choose to use an external serializer such as Marshmallow over the MongoEngine built-in serializers on Document object.