What are possible scenarios/use case where scripting language like ruby, Jython etc are used along with Java?
One use case is for allowing extensions or plugins to be written for your app using a scripting language.
Another would be to allow for writing complex filters or formulas for an application. Say you had a task management app and you wanted to allow users to write a filter so that they can create their own views on the data. Yes, you could give them some subset of SQL, but giving them access to a scripting language would be a bit more safe.
Another example of this would be in a scientific app where complex algorithms or equations may need to be created by the user and then used in the application... without having to recompile.
Hope this helps.