The only faction that does this is GNOME; KDE, Cinnamon, Mate, whomever else is fine with maintaining two different backends.

Can't speak for the other two as I have never looked into them, but KDE actually met that with hostility for a long time before caving in : https://github.com/sddm/sddm/issues/173 https://github.com/sddm/sddm/issues/465 https://github.com/sddm/sddm/pull/476

And for the exact same reasons as the ones I mentioned :

Debating philosophical merits of consolekit vs logind is nice and all but this is a case of "you want it? you do it". (feature request for CK1, pre-CK2 support, Nov 2014) (on a feature request in Aug 2015 ) @EricKoegel the problem with adding one more backend is that no sddm core developer we'll be able to test the code because we are all using systemd. It's like the passwd backend, we provide one but since we are all using pam it's not very well maintained.

So what do we do with this? Nobody on the team can test it. (on a pull request for code that adds CK2 support in Nov 2015)