Drop chatroom bookmarks and keep chatrooms inside roster
The current way the MUC submenu look is pretty awkward and I have never seen anything similar in any other GTK app. Especially the centered and underlined titles are not intuitive at all.
I'd suggest making the menu look just like the "start new converstion" item above - simply list all accounts and rename it to "join a new room". The "bookmarks" can be moved to the roster as a special group, say above transports. Using a muc_inactive and muc_active icon for rooms you are not and are in respectively. Context menu should provide you with a dialog similar to "manage bookmarks" but without the list on the left (you are editing one particular room). Similarly, joining a room and clicking it's header should give you an option to add to bookmarks or whatever you call it, displaying the same window.