Yes it is, but since "KDE 5" I've seen less of it. Besides the obvious example of Konqueror another great example of it was reKonq, which used Akregator, Okular and Kget to handle RSS and PDFs respectively (all within the reKonq window).
Now not even Falkon does that.
Just from the top of my head that I've noticed as a user: several apps, such as Dolphin or Yakuake, use konsolepart; KWrite uses katepart, and Ark uses various parts in its file preview.