The "next" button is like pressing ALT-TAB in a desktop keyboard. It snaps immediately to the next open window. When there is only one window open it is not shown, and you get blank black square. You can still add a minimize button if you want. You can also set a button "prev" to go to the previous window.
The original matchbox window manager had an option for "menu", clicking that button gives a list of open windows, like the home-key menu. Nokia decided to remove that for an application specific menu, and instead add a left panel with three app launchers, two task switchers and a hardwere button for taskswitching too !

I really want you try this button, but I'm afraid that if you don't how to do it you might damage your system. Unlike the statusbar.conf where there is a fallback file, if there is an error in theme.xml you end up with a reboot loop.