Hardly any battery life is required for animated wallpapers, unless of course someone spends all day looking at it!
Most people barely look at a homescreen before choosing an app. I once did some tests, and the animated wallpapers used less than a couple of minutes of CPU time out of an entire day of typical phone usage... sometimes far less.
This is just Apple doing their usual measured release of common features that the competition had a half decade or more ago. It not only draws some people back to iOS, it also helps pad the keynotes
One question is, will it be like the Apple Watch and only Apple-designed animations are allowed? Or will they let developers do them too.