Why MOCAP data don't use simple point coordinates? This might sound stupid for you but I really don't know how it works in the domain of 3d animation and motion capture, but I need to work with ASF/AMC files at the moment and I find it a little bit complex to understand (being a non-native english speaker does not help).

So the question is, why motion captures main file types don't use simple joint coordinates for the skeleton? 

Edit: I am particularly thinking about Acclaim ASF/AMC format mainly because I need to use it with an imlementation of an algorithm that use joint vertex positions and deltas from initial position for the motion part. Having looked at how the data are represented it makes more sense to me to represent skeletons as joint vertex and motion as deltas between the intial skeleton and the new moved one.
Joint coordinates alone don't give you enough information about rotation.

For example, hold out your arm to your side. Notice that you still have one degree of freedom to twist the arm without affecting the positions of your elbow or wrist. Thus, if you had a motion capture format that stored only those coordinates, you would not be able to distinguish these poses.