The previous entry investigated Gnome and logind, and found that practically speaking, Gnome does depend on logind. There are some alternatives, but they are not complete or well-maintained.
This entry discusses the dependency between logind and systemd.
Logind is deeply embedded in systemd. Here is a list of logind features, which seem like a grab-bag of vaguely related requirements. If anything, their relation seems to be they are goals of GnomeOS. Maybe Poettering looked at that list and tried to implement as many as possible to convince Gnome to adopt his software.
At a minimum we can say the name 'logind' is a poor choice for a name, since it does much more than that: the name doesn't describe what the product does.
Logind depends on systemd (for example) for calling unit scripts to shutdown or put the computer in sleep mode. In logind-dbus.c function execute_shutdown_or_sleep() you can see an example. Systemd also seems to be required for detecting hot-plugged devices and probably for other stuff. Note also there is a clear dependency on DBUS.
Logind is described as a tiny daemon. That's a lie, the thing has 15491 lines, with the largest file having 3023 lines (3023 lines is large but not necessarily bad).
As always, if you find any errors, please let me know. I'm trying to find the truth, not start fights.