Gedys Intraware has implemented one of their standard products 'Business Mail' as composite application in Notes 8. One advantage they point out is the ability to implement these extensions to mail now without changing the mail template.
For one thing the context data they want to display is put in different tabs which are loaded in different threads as part of the composite application infrastructure in Notes 8.
Another use case is when you implement Eclipse components that pull in data from different NSFs. There is a performance advantage since different Java threads can be used. This is not trivial since you need to clean up in the same thread where you created the session but most Notes developers who have used the Notes Java API know how to do this.