In order to avoid breaking the session, consolekit required that X be run on the same TTY where the session was registered. The ck-launch-session command could be used as a workaround, to move the permissions over to a new TTY.

However, logind does not come with an equivalent to ck-launch-session, so it is always broken to use xorg-xinit to start X on a different TTY.

The current xserverrc (provided by the package, not upstream) has this snippet:

exec /usr/bin/X -nolisten tcp "$@"

It could be changed to the following:

#!/bin/bash

if [[ -z $XDG_VTNR ]]; then
exec /usr/bin/X -nolisten tcp "$@"
else
exec /usr/bin/X -nolisten tcp "$@" vt$XDG_VTNR
fi

This will check if there is currently a session, and avoid breaking it. It can still be overriden in ~/.xserverrc, so it's just about providing a working default.

Right now every user using systemd and xorg-xinit has to work around the issue themselves, which results in a lot of boilerplate instructions on the wiki, and an endless stream of issues in irc (which can be non-obvious to diagnose - broken sound, 3D acceleration, etc.).