As of today it looks like this behaviour pertains to an older version than lightdm 1.18.3.
However on lightdm 1.18.3 I encounter that the screen illumination is switched off after
dm-tool switch-to-user userA
The computer (in my case a Lenovo T400 running xubuntu 16.04, currently with kernel 4.10.0-32-generic x86_64) is still running after that, I observe the WLAN LED blinking. In this situation I can use Ctl+Alt+F1 to switch to a console (and the screen light goes on!) which opens with a greeter dialog. But I can also immediately use Ctl+Alt+F7 to return to my GUI session which I left.
It looks to me that there is a bug in dm-tool switch-to-user switching off the screen illumination (at least on my laptop).
Of course, when I am in the console started witn Ctl+Alt+F1 I can log on as a different user and start a GUI issuing
But this is definitely not what is advertized for
switch-to-user USERNAME [SESSION]
"Switch to a user session. If this user is already logged in then
the display manager will switch to that session, otherwise a new
session is started. If authentication is required a greeter
will start to collect this. If the session option is provided then
this session type will be used, otherwise the default for this
user is used."