> Ctrl+Alt+Backspace and Alt+SysRq+K perform distinctly different operation: they kill everything in this console and start new session. Ctrl+Alt+Backspace switches to separate context where only "guaranteed secure" set of programs work
Sure, they work differently, but they solve the same problem. Think about it, what do you actually need this feature for? It won't protect you from virus deleting all files of your user. It won't help you against trojan, looking for your bank account. There's basically ONE problem that it should protect you from.
Imagine a public machine that different users can log in to. It does not matter, is it windows, linux, graphical or text terminal. User just comes, logs in, does the job and logs out, another user comes, etc. Now one of users creates a "fake-login-program" that looks exactly like a login screen, runs that program and goes home. Another user comes, thinks that it's a real login screen, enters login/password, and "fake-login-program" sends them to the author. That's the problem.
And that's the moment when you need those keys. If you're going to log in to a public Windows machine, you first hit Ctrl+Alt+Del, just in case, and then enter login/password. Same when you come to a public Linux machine, whatever you see on a screen, you first hit "Secure Access Key" (if there's something running, it gets killed, getty will respawn), then enter login/password. That's just another (better) solution to the same problem.
> (and can be used for forensic purposes without immediately killing everything or to just securely lock/unlock workstation's screen).
You can Ctrl+Alt+F1...F6, SAK, log in as root and "only guaranteed secure set of programs" will work there for you. Windows just don't have such a simple thing. :)