If I recall correctly, doesn't party chat have a limit on numbers? If you had a guild of say 12 people online at once, I don't think party chat would be enough.
Having several chat channels might be nice. One for your guild, small group channels that could be created on the fly, and perhaps a small region chat. A.i. not a chat for the entire "world", but only those close by? You could join subchannels within the world since world chat would of course go down in flames.