What's in a Non-de-plume?

A cursory glance at the names of contributors to The Artifice shows that many of us choose nom de plumes (pen names). My own pen name is a variant spelling of a character's name from an Agatha Christie novel, whilst other contributors have chosen pen names that either reflect their interests, their sense of humour or they serve as a personal statement. There are many reasons to use pen names. Some may be for political or cultural reasons. George Eliot (1819-1880), for example, was writing at a time when it was difficult for a female writer to be accepted simply as a writer and not be judged by her sex. Conversely, I recently met a male writer who writes romantic fiction under a female nom de plume; and very successfully too. Discuss how the invention of a 'literary double' might empower the writer and, just as importantly, have our nom de plumes become characters in their own right?