Model who pose for pictures should be fair game, theoretically. In practice however, this presents another time bomb for CS. If CS allows comments on model's looks , then i can see threads potentially degenerating into a free for all and eventually a tit for tat situation will ensue amongst members over time.
In a mature, controlled and professional environment, I can see the value of comments on all aspects, model's looks included. However, in the present CS environment where everyone & anyone can comment, it is perhaps best to be conservative in the interest of preserving harmony amongst members.
I am not implying that CS members are not mature/ professional .. most are mature and professional, but in every environment, there is bound to exist some who are not and it is these few individuals who normally spoil it for the rest.