Per WP-API/WP-API#924, a wide set of query parameters have or will eventually have support with the majority of built-in routes within the API. While there is no guarantee that these routes will be supported by custom endpoints, including them in WPRequest so that they are available to custom routes by default without the sue of a mixin could make it easier to interact with searchable or queryable custom endpoints and seems to have little drawback (because an invalid parameter will trigger a commensurate error and therefore be rapidly obvious, and an invalid filter will have no effect).