Actually I can't think of any, but in the JavaDoc of PlaceRequest it is stated, that an empty PlaceRequest might be valid: 
   /**
     * Builds a request without any name token and without parameters. You should
     * typically use {@link #PlaceRequest(String)} and specify the name token.
     * However, this version is acceptable when calling
     * {@link com.gwtplatform.mvp.client.proxy.Proxy#reveal(com.gwtplatform.mvp.client.proxy.PlaceRequest)}.
     */