I think the other reason is this. If you have the client set to be able to input any character set, the net result would be guilds and social circles totally based upon ethicity, region and language spoken. iRO would then not be iRO, it would become nRO - 'nationalist' RO. That would kind of suck to be honest. People would just stick to their 'own' (trust me and go check out some pservers if you don't think this would happen) and not be part of the larger community. This would be a shame, because in the 9 years or so that I have been playing this game on 2 of the official English language servers (iRO and oRO), I've learned to curse, utter wonderfully vile obscenities and even learned to say 'hello' in a variety of languages, just because I've been lucky enough to be in some very diverse guilds made up of players from many different cultures.
Like I said, the current client doesn't really stop them anyway.... so that's sorta moot. the majority of guilds communicate on their VOIP servers.
Restrictions or no, there's still group of people (some guilds) that keep their guildchat entirely in spanish/german/tagalog
Which is why this restriction is pointless. It doesn't DO anythin except annoy.