Multi-user system vs multi-uid system

On the contrary, I have on numerous times (and not just in the last few years) had bugs where various GUI programs misbehaved, and when I logged out, or restarted gdm, the processes persisted, such that when I logged back in things weren't working right. This would be fixed with a killall of all my processes from a virtual terminal, or by rebooting the computer. This new default would cause these sorts of issues to always get fixed simply by logging out and logging back in again.

Buggy code doesn't have to be malicious in order to cause problems, and you don't need a multi-user system to benefit from this change. True, you always reboot every time you log out, and maybe that is your habit if you're a Windows user. But I don't have that habit, and prefer for my computer to behave reliably and predictably, where I consider "doing the same thing each time I log in" as an aspect of reliable behavior.