title: When would I use an external serializer like marshmallow over monogoengine's in-built JSON serialization (from_json and to_json)?
body: <p>I am writing a simple REST API in Python using <a href="https://github.com/flask-restful/flask-restful/" rel="nofollow noreferrer">Flask-RESTful</a> and their <a href="https://flask-restful.readthedocs.io/en/0.3.6/reqparse.html" rel="nofollow noreferrer">documentation says</a> that they are planning to deprecate their object serialization (reqparse) in favor of serializers like <a href="https://github.com/marshmallow-code/marshmallow/" rel="nofollow noreferrer">marshmallow</a>
  My API is reading and writing from a MongoDB doc store, using <a href="https://github.com/mongoengine/flask-mongoengine" rel="nofollow noreferrer">Flask MongoEngine</a>.</p>
  
  <p>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.</p>