As stated by rmaddy in the comments, if you are supporting iOS < 8, you still need to do this:
General / App Icons and Launch Images
Launch Image Source – select LaunchImage for asset catalog.
Add a 640×1136 image for the “Retina 4” option.
So I guess the LaunchScreen.xib is useless if you target iOS < 8, unless you want to update both that and the asset catalog when the image changes.