After this second Beta, there will be two Release Candidates, one on the 10th and the other on the 31st of January. All these releases are **not** final, they are for testing and should not be used in production...
Pretty sure it is only configured like that to show off the new wallpaper through the video. I don't think I've ever seen a distro ship KDE with a rotating wallpaper (its always a static one, whether its KDE's main wallpaper, or the distro's selected wallpaper).
You can have a wallpaper slideshow. That said, the default is a change every 10 minutes and not every 2 seconds like here. But as we don't know the final winner of the wallpaper contests yet, we decided to run through the six finalists (which we do know and are public).
Oh that's pretty sweet! I've used the slideshow for a while (and have always wondered why Plasma is the only desktop I have used that lets you set wallpapers per-screen...), but I had no clue about that one!