Eh, most devs involved are volunteers. If they enjoy, say Xfce, and decide to dip into development, then it's not wasted time for them.
Telling them they're wasting their time, because KDE already has dips on the Windows-like paradigm (and, given its customizability, all the other paradigms, too), will not make them invest their time into KDE.
Clearly, Xfce is different enough for them to prefer using it, and as such, it's also perfectly legitimate for them to develop it. Not that they need legitimisation, because again, it's their free time.