Well this is a great discussion but and as I often do I will play devils advocate here and say. No Portraiture itself, is not a Art form or a movement in art. But a portrait can be art. For me portraiture is not a art form but a subject matter. Just as a landscape or still life is not a art form or style but simply the subject depicted. Picasso considered these portraits
http://www.miracerros.com/artwork/g_picasso_0.htm
Are they no less valid a portrait as a more realistic representation of the subject? Most commissioned portrait Art falls in the realist realm as that is what the person commissioning the painting wanted. But at some point most good portraits differ in some way from what a photograph can capture. It is that quality that draws you into a painted portrait I have never heard anyone say that photo gives me the creeps it's eyes seem to follow me around the room yet I have heard that about painted portraits often. I have see a few truly great photographers that have been able to capture personality on film but not many it is very hard as photographs can only capture a moment in time and light but the artist eye and hand and the medium not only captures the subject but also the personality of the artist himself. It is Not what you paint but How you paint. So I have to say No portraiture is not a art form but I also say nether is landscape painting or still life painting. But all of them can be art. I paint one way no matter what I am painting I do not change myself so my art does not change based on my subject It is still distinctly mine. THAT is what makes art any style or subject art. It captures as much the personality of the artist as it does the subject.