Yes, undergrads should be taught good and useful ideas. Graduate students however need to be taught both the good ones and the bad ones because professors need to be able to examine ideas from outside and make coherent arguments about whether or how they are good or bad. When I have met people who complain that graduate schools waste their time on things that they don't want to learn, I have explained to them that they don't really want a graduate degree, or more to the point, they don't really want a graduate education, they clearly do want the degree.