(with classmate Paul Shepherd)
A lot of things change during the animation, but the reference is a great way to avoid starting with a blank piece of paper.
(I also feel that reference video footage will soon be replaced by low-cost motion capture systems. Same idea, but the reference will automatically be available as 3D information in the computer, and then you can either layer keyframe animation on top, or use it purely as a traditional reference.)
Yet Another Example:
Reference (starring myself and classmate Joanna Griebel):