Well, as per the title. Cyrillic, Arabic, Han and whatever else usernames should be allowed.
Because we live in Unicode era and there are languages which use not just ASCII letters? Чесно!
As a usecase or whatever you call it:
On Wikimedia Phabricator installation users who have their onwiki username in non-ASCII scripts have to use a different username on Phabricator and it is partually more difficult for them, partually more difficult for others (when you want to subsribe a certain person in this case you've got to know what username they have on Phab instead of using their normal one)