it used to be there by default, but isn't any longer because:
o it took up space and cluttered the panel, which is already very cluttered and until this change didn't fit on 800x600
o this same functionality is available from the K menu
o it isn't any more obvious (due to being just icons) than finding this functionality in the kmenu
o most people use those buttons infrequently: so do they deserve such a prominent location?
o they were in a place that was easy to click on, even though they didn't need to be, esp since they result in locking the desktop or starting the log-out functionality