The original reason for blacklisting \mbox was http://bugs.kde.org/103026
It is still very unlikely that you or any user of your site could misuse \mbox or \line
Commands like \input, \include or \write are dangerous ones and in practise you can do a lot with programming commands.
I suppose you can use \mbox or \line on your site safely if you use it like it is ment to be used.