Installing and running KDE will mess up GTK settings in GNOME for instance. You might end up with the Breeze GTK theme in the GNOME session. Which works, but this is most likely not wanted (even though GNOME looks great with the Breeze theme).
I'd not advise regular users to do this without a warning.
Also, trying to chase the elusive 'casual user' is what caused all the GNOME UX mess in the first place I guess. I'm not an 'archetypical' Linux nerd, I hate wasting time with fixing stuff that should "just work", but I'm also expecting a computer to be a professional tool which I can customize to my needs (within reason at least).
The ease of the GUI ends when a serious system-level issue arises. This has never not been the case, it's just a matter of how much the documentation expects you to know what's going on, and how much that impacts the first-run experience. If the first-run is good enough, "reinstall" becomes the go-to fix.