Approximating a (solid) object with spheres means the spheres are in constant distance and angles with each other. So their trajectories are no longer linear if there is any kind of rotation.
The point isn't to approximate a solid object as several spheres, but to approximate it with one enclosing sphere. If two objects aren't close enough for their bounding spheres to touch, then you don't need to do a collision detection using the more complicated geometry.
