"Yeah but who's messages/notifications are you gonna see if there are multiple users that are logged in at the same workstation?"
When it looks like on the mockup, it really is a good question. But in GS lockscreen is not just a list of users. It's like a normal session, just locked. So there will be notifications of user whose locked session is visible. (You see YOUR notifications, YOUR wallpaper, etc. If you like it of course.) Other user can just click "switch user" and he can log in or see his own (or somebody's else) lockscreen.