I don't really think that's true. Look at Jabber.org which is probably the biggest jabber network out there and it seems they're scaling pretty well.
Edit: Also XMPP supports UTF-8 for everyhting out of the box and as far as I know it's not really possible with IRC (or did that change over the last years?). E.g. nicks with non-ascii chars are no problem to have..
XMPP MUC also has the problem that the largest XMPP client, Google Talk in Gmail, afaik does not support it. It handles multi-user chats via some proprietary method that only works for Gtalk clients.
Edit: Also XMPP supports UTF-8 for everyhting out of the box and as far as I know it's not really possible with IRC (or did that change over the last years?). E.g. nicks with non-ascii chars are no problem to have..
Why do you think it's not fun?