Half are those otherwise incapable of expressing themselves or what they stand for wanting to have a sense of belonging or individualism in a homogenising world. A cry for help, perhaps. Maybe with the advent of myspace people will feel that if they can be so easily summarised, it may as well be in a medium that can easiuly be edited, i.e. not the human skin.
The other half do it because they think it looks good, but they are usually wrong.