QT is just the graphical toolkit, as I said in the OP.
It is used for assembling the User Interface, and is embedded within calibre. I doubt you have ever thought about Firefox and Google Chrome using Gtk2 either.
These are just of interest for development.
If calibre v2.0.0 works for you, you can probably just leave it alone.
Or simply re-install the older version to be extra-extra sure. Uninstalling the currently-installed version works too.
Uninstalling calibre does NOT delete your library, nor does it wipe your settings.