I guess I look at those threads differently. Two of them give people a place where they can post pictures to get criticism and feedback, which can be very helpful I know because I lack the capacity to evaluate myself. It's always in flux how I feel about myself, and when I wasn't sure, and wasn't public, I am sure that would have been a great resource for me. I think/hope everybody understands that the judgement is just of the picture(s), and passing in a picture is very different from passing in public. But the feedback is not useless because of that. First, it's a safe place to get other people's thoughts about how you look even if it is surfacey, also you if you know you pass in a picture, it tells you that you are doing some things right. The unrealistically positive feedback also has its place because it can make somebody feel a little better when they feel so bad about themselves. More good than harm imo.
I see the other two "vanity" threads to be more inspirational, which also has its place imo. When I was first starting out, seeing people (also hearing their stories) that had made a successful transition was one of the things that helped me hold on and have just that little sliver of hope which honestly saved me. Realizing that it was possible to come out on the other side and have a normal life was so powerful for me. And you know what was, and still is, the most important thing for me to see? It's the smiles and joy that is present in people's eyes. Nobody has to be drop-dead gorgeous to show that, and I don't think anybody should feel excluded from or hesitant about posting anywhere. It's the inner beauty that is so wonderful and encouraging to see.
We can talk about the vanity threads and whether people that post there are just narcissists or insecure basketcases, and maybe... but I think that is just pessimistic.