"The polymorphic association, on the other hand, while it bears some resemblance to the regular polymorphic union of a class hierarchy, its not really the same since you're only dealing with a particular association to a single target class from any number of source classes, source classes which don't have anything else to do with each other; i.e. they aren't in any particular inheritance relationship and probably are all persisted in completely different tables. In this way, the polymorphic association has a lot less to do with object inheritance and a lot more to do with aspect oriented programming (AOP); a particular concept needs to be applied to a divergent set of entities which otherwise are not directly related. Such a concept is referred to as a cross cutting concern, such as, all the entities in your domain need to support a history log of all changes to a common logging table. In the AR example, an Order and a User object are illustrated to both require links to an Address object."

http://techspot.zzzeek.org/?p=13