But there's always the risk of someone not knowing, like someone new to the forums. However, I understand your perspective.
However, if you already have an account, why do you need to socialize with another? If everyone knows who you really are, why not be you?
I know it's a fun acting stunt, however, there's always the danger of taking it too far. As @Ariella_Newheart mentioned with Miss J, you begin to mix your personalities with theirs. I know no double user would ever intend to do that, but even if everyone knows, it's still fake. You're still talking to your friends as a fake person. Even if they know it's you, don't you think they'd rather talk to you as yourself then a pretending double?