As a KDE fan, I don't use any of these "redundant" programs, unless there is a true benefit.
The thing is that, LibreOffice works and looks great in Plasma, and Calligra doesn't do anything special.
It's even more obvious for me with KTorrent. We already have qBitTorrent!
I absolutely like things like Neochat, tho, because in this case the "official" alternative is an annoying Electron app.