Since application scope applies to all users of the web application, it wouldn't make any sense at all to put an ActionForm in Application scope, since an ActionForm represents one user's interaction with the application. The only scopes that make sense are request, which spans only a single request to the server, or session, which spans multiple requests from the same user.
If there are portions of the ActionForm that do apply to all users, it's a good idea to separate those out and put them in Application scope rather than in the form bean. For example, suppose you have a select box that allows the users to select one of the 50 US States. Since that list of States doesn't change from one user to the next, you could put only this list in Application scope and leave the rest of the ActionForm in request or session scope.