Since identifiers like usernames are seen by people they are susceptible to homograph attack and _do_ deserve to be treated a bit more carefully. Also you probably dont want usernames like ń̸̡͍̲̲̫̰̦̔͛̋̉͊̔̈̈̈́̀͑͘i̶̜̔̐̅̔̑̈̕͝͝g̶̢̭̮̲͕͉͔͙̳̥͖̉̏̇̎̊̈́̊̆̃̎̑͆̿͠ͅh̶̡̛̪͔̯̯͈̼̿͊̂̍͐͒͐͐̆̽͛̄̽͝t̸̛͔̮̆͊̋́̑̓̅̀̆͋̕ͅf̸̤̗̺̣̤̝̟̱͎̦̀͒̽̓̋̏͌͋̇͛ͅḷ̶̭̓̿́y̵͍̦̫̫̠͆͛͋̓͑͑͋̔͑́̔̽̚̚