> I don't need to imagine that. This is exactly where we use Ctrl-Alt-Del on Windows the most.
Good. Many people have a dedicated machine, often more than one, they need neither C-A-Del, nor C-A-BS, nor SAK, they just lock screen and don't worry about such things.
> this is where you conveniently change your usecase to make sure you'll win the argument. Why would I log out?
I explained a more complex case, since I wasn't sure what you actually want. What you have described is a regular switch user (http://i.imgur.com/uhIeO.png) feature that is supported in every distribution around.
To protect from someone creating a screensaver-like tool with fake "Switch User" button, you can configure display manager to autologin on tty1 and run a single program with the large "Switch User" button on it. After that to be safe every time you want to enter the password you press Ctrl+Alt+F1. :)
> Windows received this ability in Windows Vista which is six year old by now!
That's another ability. Under Linux on any virtual console you can run arbitrary "guaranteed secure set of programs", anything you want, not just winlogon dialog.
> Both Linux and Windows continue to evolve and while some places where Linux is better still remain Windows is better in many, many aspects.
Windows is usually better if you got used to it and its bugs. Windows is also often better when it works right out of the box and does exactly what you want. But when you want more, or want to optimize it for your needs, it's easier to configure linux then fight with windows. IMHO, of course.