If you only use "simple types" then you often get implicit assumptions about the "values". For instance note that "web-services" basically means you have a very simple interface defined by the HTTP-protocol.
But what is actually inside the HTTP-payloads can then have many constraints on them which are not declared anywhere. For instance your code might assume the payload is JSON with several required fields in it.
For that you have the "closed for modification but open for extension" principle.
If the server is new and fresh, yeah it's ok to assume the payload of a request will be a JSON object with some required fields, but leave the parameter there in case someone decides they will start sending XML payloads to it
