There is a difference in simplifying access to commonly used objects and simplifying the creation of tests.
There is not a lot of setup to creating an org.springframework.mock.web.MockHttpServletRespon se object. In fact it's as difficult as:
You can then set your content type, headers... and have access to the output stream. I could potentially see a little bit of value in being able to get the OutputStream directly as a method parameter, but then you'd have to have at a minimum two other parameters to be able to set the content type and headers.
HttpServletResponse response = new MockHttpServletResponse();