Blender's motion paths are limited, but can be made into a very powerfull animation tool.

The motion paths in their current state are just for visualizing the path of a keyframed animation, they are not for editing an animation.

To improve the tool the following problems need to be tackled:
1. motion paths cannot be edited like a cuve.
2. motion paths dont update automaticly, you have to manually recalculate to see change.
3. And worst of all, to make any change you have to clear the motion path (delete it) set new keyframes or edit existing ones. And then recalculate the path again to see anything.