Some days ago I tried to create a user account for myself on wikimania2007.wikimedia.org. But I receveid the message ''Login error: The name "555" is not allowed to prevent confusing or spoofed usernames: Does not contain any letters. Please choose another name.''. I'm a active editor since July 2004 and use the User:555 in lots of wikis since 23 May 2005. [[:en:555 (number)#Other fields|Wikipedia have a article devoted to the number 555]].
Is really necessary this in AntiSpoof extension? I don't imagine any vandal naming in numbers-only usernames, but something like ''User:5559854123 is the phone number of the mother of Jimmy'' is possible using numbers '''+''' letters.