I first noticed this when gnome was trying to pull in poppler-bindings, which wanted qt3 and 4. But it may happen on other packages as well. I had -kde -qt in my USE flags, but poppler-bindings wanted not one, but two versions of qt. An emerge -pv showed that even though I had -qt set, -qt3 and -qt4 weren't. Shouldn't -qt also disable all versions of qt, and not just qt in general?
No, it shouldn't. USE=qt doesn't exist any more (except for two remaining ebuilds).