A screen name is something you use to communicate with friends and the internet at large. It is who you are when you sit down at the keys and get to spreading your word. Some people are more than comfortable to step away from this screen name and get out there into the real world. Afterall, that screen name has done nothing but chat on AIM, Yahoo Messenger, or something else along those lines. Sure it might have a few frags in Counter Strike, but unless you are playing competitively, no one really calls you by THAT name.
There are the other people that dwell on the internet and use it for socializing or a means of communication with people in the outside world. Social networking and building communities and websites with a “screen name” can draw away from what you have actually accomplished and steal away some of the recognition you deserve.