Sure, if a package owner decides that his objects will not be available for search, notification and categories then this is perfectly fine to not implement the service contract and maybe even not to name the object (like in acs_named_objects - see other thread). I just assumed that most of the packages do indeed contain searchable and categorizable content so there would be a need for the search and categories package to display the object names and provide links to the objects. And I think that such functionality (object names and object links) should be provided by the core so that packages could rely on the interface to be there - if it is being used totally depends on whether the site-owner mounts the search or categories package.
