To give people some idea of how practical a non-systemd implementation of logind's dbus API is, here's the documentation for it: http://www.freedesktop.org/wiki/Software/systemd/logind/

It's big, gnarly, essentially undocumented, and contains a bunch of features to support tricky cases like multiseat machines (systems with two or more sets of keyboard, mouse and monitor, each with a different user) that are rarely used but still need to be supported because the API's designed around them. Even the systemd developers don't think a reimplementation is feasible.