I agree that a more modern init system is necessary, but the idea of systemd is meh and the implementation is subpar.
The maintainers of Arch Linux have given an explicit list[1] of the reasons why they decided to introduce systemd:
0) it is hotplug capable
1) we can know the state of the system
2) it is modular
3) it allows dbus/udev to go back to doing the task they are meant to do
4) we can reduce the number of explicit ordering dependencies between daemons
5) we get a lot of security/sandboxing features for free
6) systemd service files can be written and distributed upstream
7) systemd is a cross-distro project
8) logind will finally deliver on what consolekit was supposed to do
9) systemd is fast
There are other things that are meant to be left around, as that's their purpose, such as screen and tmux.
I would say the safe solution would be to kill everything in the user session at multiple levels (GNOME, and user session management demon like logind, etc), and to provide a defined safe way for the very few programs that want to stick around to do so. That seems like what systemd came up with as well.