Miklos Szeredi, one of the developers working on the cross-rename support, explained the motivation on the mailing list patch series, "The purpose of extending rename is to add cross-rename, a symmetric variant of rename, which exchanges the two files. This allows interesting things, which were not possible before, for example atomically replacing a directory tree with a symlink, etc... This also allows overlayfs and friends to operate on whiteouts atomically."
With the set of 11 patches there is EXT4 support for cross-rename. Implementing support for other local file-systems is also an easy process since the heavy lifting is done by changes to the kernel's VFS layer, but additional work is required for cross-rename on network file-systems.