Why do you assume that's not my longer term plan?
I already layer things ontop of IRC such as typing notify, recent chat on channel join etc...
IRC is a simple, solid chat protocol. It's about as ugly and broken as HTML IMHO.
You don't get users by forcing them to use some swanky new protocol/system off the bat. That's a recipe for quick failure.
You could probably change Mibbit to use the MibbitChat protocol and most of your users would never care.