This will enable us to get rid of the kdelibs fork which is a pain in the ass for packaging (relevant i.e. on centos 7 (which ships kdelibs 4.10.x). Clearly not an upstreamable change, but the effort is relatively small, and we save a lot of effort on the packaging side.
We simply have to take the latest version from our branch, copy it into kdepimlibs, rename it to PIMRecursiveFilterProxyModel and change all uses of the model in kdepimlibs/kdepim-runtime and kdepim.