I personally prefer Ubuntu login screen to Gnome lock screen, because the former is more beautiful. Also one may prefer it due to wallpaper privacy, for example on an encrypted home.
Well, after months of searching (and dissapointing) I found a workaround today and just want to share:
It’s easy through Dconf Editor. Open the schema org.gnome.desktop.screensaver, enter the command
dbus-launch gdmflexiserver -xnest for the variable embedded-keyboard-command, and then check embedded-keyboard-enabled for the same schema.
Now Ubuntu login screen appears every time the system wake from either screensaver (lockdown) or suspend or even switch user.
I hope this will be default with Mir.