Extrac: To be fair not everyone created their username with the purpose of it being a multiplayer handle or one they would share with the world, just an account name they would remember. I know I would feel a little uncomfortable if my account name was Firstnamelastname or the first part of my email address and was forced to broadcast it.
Another aspect is that forming groups/clans becomes a bit harder when you can't put clan tags in front of your name, so can definitely see the plus of a changeable online handle.
These are some of my reasons for asking. Another reason is that I like to pick a username that suits the game I'm playing. I tend to use a different name for sci-fi and fantasy games.
SpaceGnome: It is possible to change your username by contacting support. I do agree though, we need to have an edit option for our usernames via settings. We would be able to change our names and it would lower the amount of tickets support gets. Win-win for everyone.
I agree, it'd be favourable if we could just change our in-game username ourselves.