It would be really helpful if I didn't have to manually join the rooms I'm interested in every time I launch chat. What I'd suggest is one of two mechanisms for auto-joining chat rooms.
Make a setting so that you will automatically join rooms you mark as 'favorites'.
Automatically join the rooms you were in last time. (I'd prefer this one)
This way you don't have to reopen all the chat rooms you want to be active in, you simply log in and they are already there.