In 5.x (when signed into the web-client /zcs) if you opened a chatroom in an external client (pidgin, spark, adium) even with a different /resource it would open in ZWC. Then chats would be logged. (Even non-chatroom messages were sent to all clients.)
Just noticed that literally the day our internal servers were upgraded to 6.x my chat logging history stops, I think we might have started doing individual resource routing for IM or 'by which ever client last communicated' though I haven't looked hard into it. Signing into a chatroom on an external client doesn't open it in ZWC (hence no logging).
Now whether these RFE's will handle that I don't know (still have logging in my external client but it would be nice to aggregate in my server's chats folder):
Bug 21761 - Legal intercept support: IM (typically just ZWC,
SMTP/IMAP/POP isn't included in
Legal Intercept - Zimbra :: Wiki ) &
Bug 12697 - Other collaboration data (IM, Docs, Calendars) need to be archived (
ZAD)
Regardless, the correct behavior for normal messages should be broadcast, but subsequent replies go to the same client resource I've answered from. Currently this does not happen, so there's an RFE/bug to be had there.