Jean-Jacques Dubray just triggered a thought with a comment he made on Twitter regarding the rough ideas to adopt MVC 'ideas' in JAX-RS 2.0. He was worried that an MVC based approach to creating representations exposes the model to the service consumer.
Interestingly, this is only possible if the service uses generic media types. Specific media types sufficiently limit the kind of information that can be expressed in the representations and prevent the service specific model to be visible to the outside.
Specific media types enforce the resource boundary and prevent the service's domain model from leaking.
This is probably a much better argument against application/xml or application/json than the usual reference to the violation of message self descriptiveness.