What breakage does this actually fix?

This change actually solves a problem that I have with Gnome for quite a long time (since 3.18? if so, it is already one year old): sometimes when I try to shutdown my system, it simple hangs waiting for some process to finish. I thought this was a systemd bug, however after investigating a little I found it was some Gnome hanging up when asked to finish, and systemd was painfully waiting it to finish.

The workaround was to reduce the time that systemd waits for a process to be killed, however this is a more definitive and less hacky solution, at the cost of changing on how you think about *nix logins.

And yeah, this is a problem with Gnome that should be fixed. However it is the kind of problem that is intermitent, does not occur with everyone, and come back after a while (I didn't had this problem in some point update of Gnome 3.18, got back in 3.20). And I remember to had this problem in KDE too, even before systemd existed. So yeah, an old and annoying problem.