Portraiture is actually a very old form of art. As a matter of fact, for the majority of art history, art was not based on the personal expression of the artist, but on an academic training in art. Art has primarily been considered the competent use of tools - i.e. paint, brushes, clay, stone, chisels, charcoal, camera, etc... to represent something. It has little to do with the notoriety of the artist, and much more to do with the skill and the process. Artistic expression and the concept of art for arts sake, is really a 20th century invention. This is not to say that artistic expression and the creative choice of the artist was not a part of art before this, it simply was not given the same attention we give it today.
The type of portraiture in the above example was very common in the centuries before the camera. These types of portraits were a way to document a person's personality, status, and interests, as well as how they looked at a given time in life. Family trees were documented, the growth of children, important life events, and sometimes even death. There is much to learn about culture and history from portraits, as well as the personality of the sitter.
As for your question about digital portraits as artwork, I challenge you to look through the work of local photographers. You will notice a huge variance in the level of skill in posing, lighting, scenery, and artistic expression through the medium. To take it one step further, compare the pictures of these photographers, especially the better among them, to a generic photo studio or a home snapshot taken by a family member.
