But I do not want use any letters. This thing is weird. Are this improves something?
I don't get your problem with using any letters? Why is that an issue?
I think they just wanted to have a name like '1337'[1], but even though it was free the rules for usernames somehow do not allow that. I dont know why exactly, but I know - as I posted above - there had been some issues with account names recently and that might have resulted in overly strict rules. It might also be totally unrelated and the rule is in place longer.
[1] that user actually exists ->
https://bitcointalk.org/index.php?action=profile;u=10704