I don't feel that adding labels or categorizing members would help discussion in the least - in fact I think it may hurt discussion.
I cringed a few days ago when someone responded to one of my posts by stating, "I'm not sure whose side you're on." All I had tried to do was clarify a point of science for this person, and now they were essentially asking if I was friend or enemy. I think it may be less than constructive to label people as such from the start.
Also, I could see people dismissing each other's now-labeled ideas before reading them; that is, evaluating the world-view of the person making the comments rather than the comments themselves.
There are other forums on the net that use such a system, and it seems downright silly to me (especially one that gave two choices for religion - Christian or Atheist). The discussions on those forums seem much more adversarial, though they also don't have the same quality of moderation found here. I refuse to join such forums, usually because I refuse to label myself, especially inaccurately.
In this way, it is immediately easy to tell what position each person subscribes to, especially when it isn't always clear from the post.
I think this statement bothers me the most -
it would not be "immediately easy to tell," it would be immediately easy to assume. Why post or respond to posts when they are unclear? And why should we assume we know what someone's ambiguous post means because of a one- or two-word label under their name?
Your statement is exactly the problem I have with labeling members - people will prejudge and argue the label rather than closely examining the point of discussion.