I think for the most part the discussions are genuine--and that's what I come here for--even if the identities created by the posters are not.
I would only care if a member is the person s/he says s/he is if I was going to get into a personal relationship with him/her, which I don't see happening.
Some people use fake identities for the same reason we don't use our real names and most of us don't post face pics--for privacy, safety, and security. But for the most part, I think the the thoughts, ideas, and opinions expressed are genuine.
Sometimes it's fun to pretend to be someone else, to escape from your own identity for a while, or to express an aspect of you personality that is normally internalized. Kids do it all the time. That's one of the reasons Halloween is so popular and enduring, and why people like costume parties and masquerades. Some people play Dungeons & Dragons or other role-playing games (note I deliberately said "role" not "roll"), but that's not everyone's cup of tea. Some men are drag queens.
Movies like Mrs. Doubtfire and Tootsie, depict men who each adopt a fake identity and, in the process, discover another side of themselves. In the process they build real, genuine, meaningful relationships with others. Unfortunately, those people do get hurt by the deception. I remember the scene at the end of Tootsie where the female love interest says to Michael (Dustin Hoffman's character), "I miss Dorothy" and he says, "she's right here."
I think Internet forums and chat rooms are a good place to experiment with different roles. However, since some degree of deception is involved, the ethics can be a bit tricky. I don't think people should misrepresent other peoples pics as their own. If you get into a personal relationship with someone, the deception has to end, and it may be difficult to impossible to rebuild that person's trust. How can they ever be sure you're not creating another character? If you're on a site to build relationships, then that's probably not the place to experiment with an alter ego.
As far as I'm concerned, if you want to be my friend, you need to show me who you really are. But if you're just here to contribute to the discussion, adopt whatever identity you want.
P.S. I also want to agree with rob_just_rob's post, particularly the last paragraph.