For many types of movie shots, the default motion blur included with cryengine, is enough. But many sequences require a higher quality 3d motion blur. For example rotating objects, or blades of grass or leafs passing rapidly in front of the camera. Without a 3d motion blur these would look bad.
Or imagine a tornado made with high speed particles. I will upload a 3d motion blurred cryengine tornado movie in the future.
These insects look bad without 3d motion blur.
gif animation comparison
longer mp4 movie http://img684.imageshack.us/img684/6955/y33.mp4
More proof (low quality for now)
animated gif :
Panning the camera behind close objects like foliage or railing or other objects would look bad without a 3d motion blur.
This type of filming usually occur in most movies at some point and if you don't have a raytrace motion blur you will get the artifacts shown in this comparison.