Skeleton

Anatomy
A skeleton is made of two things: bones and joints of either micro or monolithic types.

Monolithic joints
Monolithic joints (or combijoints) are joints composed of muscle which binds around the bone ends. Initially these were nobly lumps that were approximately square but as Necromancery progressed they shrunk to the point where virtually all the muscle is between the two bones and only small connecting strands remained hocking on to either side.

Micro joints
Micro joints use in bone muscle and so only require non movable attachments in the joints.

Effectiveness
They benefit from using magic only in specific muscles as this allows any functional joint to continue to operate even when hacked of the main body (well, skeleton but you get what I mean). While technically possible in a zombie it is far more easy in a skeleton as there are fewer muscles (at most as many as there are joints and often less so as joints could be immobile (as feet often are)).

Further there is a much greater supply of skeletons than zombies in any condition especially not fresh ones which do not require modification to be transformed into fully functional creatures. Further a mainly decomposed body can actually require more work than a ground up skeleton build to become functional.