Another thing to consider is that CG for faces is far more difficult and expensive than CG for changing the size of actors or rendering full models. Most people recognize facial CG immediately because we look at faces all the time and are very attuned to their subtleties. So to do it convincingly is very time consuming. Using CG to alter Holm's face for every scene in 6 to 8 hours worth of movies would simply not be feasible. They'd do better to make him from scratch like Gollum.