I'd wager they want to keep "clutter" to a minimum at every screen. As in, at your lock screen you have the simple backdrop and some useful notifications as needed. But when you want to enter your password, you only need to enter your password and so the extra information goes away. I don't really care much either way they do it.I just don't understand why the slide screen is there. Slide screens are usually on smartphones and some tablets, mainly because (and more on the former than the latter) so that if the screen is accidentally, unkowingly in contact with a surface, the lock screen acts as a barrier so that you don't accidentally do stuff. (Basically the smartphone equivalent to stopping the infamous butt-dial.) Now on here, since it's for laptops and desktops, there's no real reason to have a slide screen, and it seems to be odd enough, seeing as this interface will be used with a mouse and keyboard, instead of fingers on a touchscreen.
Besides that, I just honestly don't see the appeal. I love eyecandy, especially good UI design, but I just don't see the reason for this. Sure, it looks good, but it looks no better to me than the login screen on Ubuntu/Unity 12.04. Just seems like there's no real purpose or reason for it.
Edit: For some more speculation, since they want to give you the option to switch users from the lock screen (assuming things do look the same for that, I think the video shows the real login screen), this separates the Lock from User Management. Especially useful if there are many accounts for the computer, they won't have to hide those under another button.