I agree that these should not be included.
The problem is that this type of integration between different DEs is always very fickle and constantly in need of being revised. I think they are cool nerdy oneshot exercises for a particular set of versions of both DEs, but that can quickly go sour with an update to either. And consequently are a real source of system instability that should not be encouraged on a general index, regardless of the current warning on that index. They are, of course, otherwise always welcomed as independent posts on the Tips, Tricks and Tutorials forum.
On a more personal level, they constitute a type of experimentation I no longer practice, for a few years now. There is simply no way I can assure a stable system and any victories are always short-lived. And because of the way DEs tend to converge their features, the small benefits of integrating a tool from another DE for a small set of its added features are simply not worth it. It is not just a matter of being constantly forced to finding new ways of maintaining the tool integrated. Oftentimes the effort has actually to be abandoned because the updated DE made it impossible, reducing to naught a history of previous efforts and the workflow benefits they were providing. The MATE desktop in particular was chosen by me exactly for its enduring workflow coming back from the Gnome 2 days and mimicking the traditional desktop paradigm that predates it. So how can I justify these type of integrated tools, if sometime in the near future I am forced to abandon some workflow because I can no longer integrate a tool?
I find theming a more interesting proposal. Bringing both my MATE and Xfce desktops into a completely identical look is more exciting to me (*) than trying to integrate a Xfce tool into MATE or the other way around.
(*) although it can be equally frustrating sometimes, because of certain DE or GUI toolkit maintainers decisions. But that's another topic and a rant I won't go into.