It’s also always good to have an abstraction; different types of interface may have different ways of indicating that they’re busy.
Of course, there’s also a school of thought that says an application’s human interface shouldn’t ever actually be “busy” in any sort of blocking fashion, but that’s just moving the problem around: No matter how fast a system may be, perform a significant operation on an arbitrary graph of objects can always wind up being slow enough that it blocks the user. But how an application’s human interface represent this may need to vary substantially. (E.g. by dimming out the representation of a graph that’s being processed and therefore can’t be manipulated.)
-- Chris