If the Application is split in to different modules and if the flow goes to the JSP, with an <html:form> tag, an exception such as Cannot retrieve mapping for Action /action can be thrown, if the <html:form> tag's action path is referencing an action in a submodule. The reason being <html:form> tag looks for the ModuleConfig first in the request, if it doesn't find it there, it retrieves from the ServletContext. Since, the tag cannot know the module the action belongs to it retrieves the default ModuleConfig and so the exception.
Link only to Actions pattern gets rid of this exception, since the corresponding ModuleConfig is placed in the request by ActionServlet (or RequestProcessor). One more reason for linking only to Actions.