I think it's a really bad decision. I always thought very poorly of the platform, but the username system seemed effective enough at making many people share the same name without ugly xXx_user_xXx abominations, and it was also very distinctive compared to other similar websites, very hard to mistake it for a twitter handle for example. Asking hundreds of millions of users to change their username (which only one in ten thousand at worst will keep, ignoring the inevitable massive abuse incoming) is exceptionally lame. They acknowledge in the blog post users will just add numbers or find combinations with symbols that work, too. The only benefit seems being able to arbitrarily set your display name as anything you want, which makes up for ugly usernames, but that's all there is to it?