[laumars]

I think it's just to stop name spoofing using unicode characters. ie pretending to use a registered nick by using characters that look similar. As a non-unicode example: "Bob" looks like "B0b".

Personally though, I'd rather welcome that kind of abuse but not exclude other people from countries who don't use the Latin alphabet. As demonstrated above, it's impossible to automatically moderate username abuse entirely anyway - as you can just start using non-unicode characters or even punctuation such as periods after the username. And if the chat protocol is built in a secure way (which this is not), then there should be other mechanisms for ensuring that Bob is in fact "Bob" and not "B0b" nor "Bob."