Ok, so to be more specific about use cases, I have a control which fires off AJAX requests using SPServices for data about a list schema, about content types, and about items in various other lists. Right now, I have a lot of ugly, poorly maintainable code which causes some logic to run only when all of the requests have come back successfully. By using JQuery's Deferred object, I can use the .when() method, which is a heck-of-a-lot nicer, and because it's part of JQuery its very maintainable.
It's very common that you'd want to parallelize a bunch of AJAX calls. JQuery's Deferred makes doing so a lot nicer looking than doing this manually.