Comment 25 for bug 315865

(In reply to comment #14)
> The average user will eventually run a tty. If they do not run tty, the first
> time they upgrade xorg or kdebase, kdelibs or qt, they will be running to the
> forums and will be told to uninstall the packages mentioned above and then
> reinstall them in run level 3. So the short answer is yes, they do run tty
> sessions.

You're wrong about this. K/Ubuntu manages to do and finish the upgrades without letting the X server die. Also, debian-based systems don't use runlevels properly, so that "runlevel 3" and tty-based login unfortunately aren't synonymous.
In short: your average Kubuntu user will not likely or ever use a tty.
They can, however, shut down in the middle of an upgrade, leaving their system unusable. KDE can and will not stop this, and KDM can't and shouldn't try to. A user who restarts the system in the middle of an upgrade is in potentially big trouble, and it certainly isn't KDE's job to babysit them.

Then there is my use case: When I trigger a shut down on my laptop I often don't have the time to wait to see whether there will be a prompt. And being stuffed in my backpack as it usually is in this case, having it running will cause hardware damage due to overheating and prolonged exposure to vibrations and shocks (i.e. running to catch the next train).
Whether or not default behaviour should be changed to shut down after a period of time is a different matter and should be discussed in a separate bug report.