I can't really see the point in not having the delta information kept in the mouse move event. Sure, you can calculate the delta yourself by comparing the new position with the old, but this doesn't work when the mouse position is limited by the screen/window border.
There's a lot of instances where you use the mouse for movement rather than being a cursor. First person look around views, as one example, and there's a few many more.
Besides being limiting by not having it, it's also a question of convenience and clarity. The mouse move event is usually a single instance, and hardly performance intensive, so why not just include the deltas with the mouse move event instead of letting the user hassle with that detail?
I saw an earlier post on the subject, but it had no replies for some reason. Hope this defines the question more clearly.