its nice especially when u want to reuse ur animation,
in our project we NEVER animated, even when we needed new motions for any character, we looked thru past episodes and found similar movements for any character then we merged and blended the motion data in story window,
it sounds weird for any keyframe animator but believe me, it SAVES a lot of time,
you not only can blend between different animation data but between MOCAP and keyframe data too, and the best thing is you can do that without destroying any of those data,
and for dynamics , i can say that motionbuilder is built to be
REAL TIME CHARACTER ANIMATION package, so it can do it best only,
and ya, scene opening sometimes becomes scary and worst thing is it doesnt give scene handling control like maya, in maya you can look in scene graph and delete unwanted nodes , while Motionbuilder is not much transperant ,
but still, its great package with some clever ideas,