Also, concept of XenForo UI:
Showing the version number to your visitors gains them nothing, so it's not there. Same concept as the rest of the XenForo UI.
Of course, you can say, this is a problem the admin, who has not protected the installation folder. But guys, we can speak long about what should be the admin to do and what not, but less such admins (bad? unprofessional? lazy? - nevermind) will not.
I think it's not difficult to make a condition that version was shown only to admin?
PS: I myself rarely hide the version of the forum, but if this concept of XenForo...