While this may be a 'debug' feature it sounds useful for when a fullscreen app locks up. If not these key combinations, what are you intended to do in such a situation?
I'm fine saying that, for a lock screen, you need to reboot. For any other fullscreen app, I'd expect Ctrl+Alt+Fn to take you to a tty, where you could kill any apps.
A regular fullscreen application (such as MPlayer, VLC, Chromium, …) does not grab the keyboard and/or pointer. However, applications like pinentry-gtk (for things like entering your GPG passphrase) or screenlockers do grab the keyboard and pointer.
So, in short: no, this doesn’t seem useful in that case. And what you are usually intended to do is use the shortcuts of your window manager to kill the window/application or switch to a different workspace/desktop with a shortcut and kill the process from there.
A regular fullscreen application (such as MPlayer, VLC, Chromium, …) does not grab the keyboard and/or pointer.
Regular fullscreen games do, however. If there were some way to distinguish between a lock screen and a game, it would be worthwhile to keep the feature around.
Indeed -- I've often manually set AllowDeactivateGrabs in xorg.conf just so I can use ctrl-alt-/ to kill poorly written programs that XGrabKeyboard() then freeze. Of course, my workstation is physically secure so I don't need the screen lock.
AllowDeactivateGrabs, and its sister option AllowClosedownGrabs which enables ctrl-alt-* to close apps with an active grab, have long been known to be a security hole that the user must explicitly enable. I'm surprised distributions are now somehow shipping with these options enabled! :/
