You said
"If you install just kde-core, then you get the bare minimum to run
KDE applications from Gnome."

If i'm using ubuntu without kde-core - install first say Kopete and
then say install Kmail. Would the common dependencies in Kmail be
installed again, i'd guess not - if that's the case i'm not sure that
i see the point in kde-core. I thought the package manager would check
to see if each dependency is there and install as necessary.