Not meant to be picky, but just to clarify: USE="-qt" doesn't guarantee you that the qt package is not going to be installed. It just guarantees you that if it's optional for a given program, then it will not be linked against qt. However, if you install a program that needs qt (and that means it's *required* and not *optional*) then qt will be installed anyway. It doesn't matter if you USE="-qt".
It can also be used to make sure you don't get a functionality you don't want. Say you run gnome and don't want all those Qt-packages, you can add "-qt" to you USE-flags to make sure packages don't fetch them.