The main problem with having graduated opinions is that you can only express them with one shade of candidate: it's either the Republican or the Democrat candidate. You can not nuance your view of them, and so, shortcuts are taken, since by voting for one or the other, you endorse them, whether you fully or only partially agree with them.
If the political system allowed for a more nuanced voting (Such as grading candidates for example), then people wouldn't have this dual view of the other party's supporters.