They are just different approaches. jBPM pageflows are more powerful, but more difficult to use. Many problems can be elegantly solved with either model.
My rule of thumb is to use a pageflow where there is a natural flow (often some sort of wizard) - the user wants to be able to work their way through solving a complex problem. On the other hand pages.xml is brilliant for even the most complex (in my book lots of tabs) CRUD.
I really like being able to start a pageflow in pages.xml :)
Yes, these are the intended usecases. Note that pages.xml is RESTful, pageflows are not.