Steps to reproduce:
1. copy latest Raspbian Jessie Lite image to sdcard
2. install updates, reboot
3. install lightdm package
4. use raspi-config to swith from cli to desktop
5. reboot

After these steps cli is only loaded and lightdm service shows only this:

    sudo systemctl status lightdm
     lightdm.service - Light Display Manager
       Loaded: loaded (/lib/systemd/system/lightdm.service; enabled)
       Active: failed (Result: start-limit) since Thu 2017-03-30 11:05:44 UTC; 39s ago
         Docs: man:lightdm(1)
      Process: 854 ExecStart=/usr/sbin/lightdm (code=exited, status=1/FAILURE)
      Process: 849 ExecStartPre=/bin/sh -c [ "$(cat /etc/X11/default-display-manager 2>/dev/null)" = "/usr/sbin/lightdm" ] (code=exited, status=0/SUCCESS)
     Main PID: 854 (code=exited, status=1/FAILURE)

    Mar 30 11:05:44 raspberrypi systemd[1]: Failed to start Light Display Manager.
    Mar 30 11:05:44 raspberrypi systemd[1]: Unit lightdm.service entered failed state.
    Mar 30 11:05:44 raspberrypi systemd[1]: lightdm.service holdoff time over, scheduling restart.
    Mar 30 11:05:44 raspberrypi systemd[1]: Stopping Light Display Manager...
    Mar 30 11:05:44 raspberrypi systemd[1]: Starting Light Display Manager...
    Mar 30 11:05:44 raspberrypi systemd[1]: lightdm.service start request repeated too quickly, refusing to start.
    Mar 30 11:05:44 raspberrypi systemd[1]: Failed to start Light Display Manager.
    Mar 30 11:05:44 raspberrypi systemd[1]: Unit lightdm.service entered failed state.