But drawings done from photos are just copies of what is already there. They lack the expressive quality of drawings done from a live model where the artist can really show their skill in drawing and bring something to the drawing that is beyond just merely copying a photo. Anyone can do that as the photo has already made the tough choices for you.. its already converted the model from 3D to 2D and already decided all the values for you. Thats why drawing from life is so difficult, you have to make the decisions about proportion, value, line quality and turning the form. Thats when you really learn how to draw.
If you are interested read my journal article on life drawing, its got some great input from alot of artists about how beneficial life drawing can be to the skill of an artist. Maybe sometime you will try it