*buntu are mainly beginner distros. They work fine out of the box, but many long-term users don't like them for ideological reasons.
The main advantage of Ubuntu over any other distro is that everything as an Ubuntu guide. The same is not true for Kubuntu, and if you stay in GUI, Ubuntu and Kubuntu share almost no similarities. The settings, the pre-installed default apps, all that differs greatly.
Thus the main reason for using *butnu is gone when using anything else than Ubuntu.
Which kinda sucks, because I like KDE much more.