I added that, because some of the tests respond without html at all, f.e. the first two here:
https://github.com/phpbb/phpbb3/blob/develop/tests/functional/fixtures/ext/foo/bar/controller/controller.php

Also if we get the API soon, we have to face json/xml stuff, but yeah, I think we can just rename the method to assert_response_html() which calls one method to check the status code and checks for doctype itself