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.
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.