open_basedir only restricts file operations to files and directories under a specified directory, but you can still user system ("vi /home/somedir/somefile"), so safe_mode still has a place here as it is much more restrictive then open_basedir.
Also, to reply to someone who said that 'above' and 'below' was a matter of perspective, sure it is. Of course, a file is not under another one, etc, it just pointed by some inode. But in the common language we consider the root (/) to be above everything else, and /home is below root, and /home/myfile is below /home. There is no written standard, but most people (those I know anyway) agree on that syntax.