In the current kernel, the View subscriber is hard coded to handle HTML and Core's flavor of JSON. This needs to be made extensible somehow to allow the addition of additional supported types, responses, etc.
At present it does not simply attach arbitrary view listeners for each output type because Fabien recommended against doing so, and that we make a single listener that has its own extensibility. That makes it a bit more encapsulated as a self-contained library rather than something that can only work when attached to Symfony's listeners. (Symfony's architecture strongly encourages free-standing fully self-contained libraries and thin bridges to link them to Symfony itself, which I agree is an overall good model.)