One effect of this is for example, is inability to control PulseAudio via D-Bus from X
session (although most programs use libpulse API that works via sockets) or lack of access
to user D-Bus services (e.g. org.freedesktop.Notifications) from user systemd services.
This line was added in pre-systemd times when D-Bus session has been launching together
with X server. Now D-Bus is launched by systemd on login, and unsetting
DBUS_SESSION_BUS_ADDRESS in startx results in another D-Bus daemon.