Jane Doe gets married and changes her name Jane Smith. She no longer wishes to log into your system as janedoe and would prefer to login as janesmith. Certainly one could make the argument that you should just tell her that the name can't be changed and she'll have to just live with it. Depending on your business, that may or not be acceptable.

Even worse, Jane Smith decides she hates that bleep John and they get divorced. She really won't want to see Smith as part of her username anymore.

If identity is a concern, maintain a list of old usernames. But in the real world, there are always extenuating circumstances in which allowing a username to change is good policy. Admins could blindly enforce a "never change usernames" policy, but why not just use a meaningless key and avoid the risk?

I am glad that twitter, github and stackoverflow allowed me to change my username. You could call it 'display name'. I wish hn would allow it.

You're ubinator. Unfortunately, you come home one day to find Mr. Binator in bed with some 20-something blond hussie.

The divorce paperworks sails through. Do you still want to be ubinator, reminded of that bastard every time you have to log in, or would you prefer to go back to umaidenname?

We both know where HR would stand on that one :V

really depends on the site, several places let you change your username.

Suppose my name is "Paul", my username is "paul". Now I've just transitioned and I'm not "Paul" anymore. What then?

Someone else wants to take over my account. Forever and ever they have to log in using my name?