There are quite experienced users who keep the automatically assigned username. For example, see [here](https://askubuntu.com/users?tab=Reputation&filter=all&search=user) or [here](https://stackoverflow.com/users?page=9&tab=reputation&filter=all&search=user) or [here](https://superuser.com/users?page=2&tab=reputation&filter=all&search=user). 


As you can see in the links above, there are many users with >1k rep who have kept their randomly assigned usernames. If that's what they want, why should we pester these people to change their names? This is not a social site, it's a Q&A site. Some of us like to engage in the gaming aspect and treat it like a social site but that is not its objective and we shouldn't try and force it.

In short, let people name themselves as they will.