I created an alternate username once on FCF, but I think I only posted to it once or twice. It was too much work.
I think I did it to express a view that I felt I couldn't under the username I had, since several people knew my real identity. Can't say for sure, though--it's been too long.
I think the advantage is that if your "real" identity is known behind a certain username, you can gain back your anonymity by creating a second username.
Sometimes folks do it just to have fun. But overall, for the sake of conversation, it's a lot easier if there's just one of each person. Considering the fact that most of us
don't use our RL names, I don't consider it necessarily dishonest, except when used as a means to manipulate the conversation in one's favor.
I do remember an issue with a person called Xerf...I thought THAT was a heinous use of internet anonymity and a gross underestimation of the virtual relationships that are formed, even among anonymous users.
I also think that when a person is banned for bad behavior, it is immature to keep popping back up under another name, like a child that can't sit in the corner for 10 minutes.