I still remember in one signals class, the guy next to me asked how I did for one of the homework questions, and I told him I didn't do it because it looked awful. He told me it took him several hours to solve.
"[First name], it's worth 1/2 of 1%."
"... you son of a bitch."
But hey, what do I know, I've just got an engineering degree on my wall here next to my PE certificate.
Of course you are right, that is how you pass exams. I was actually quite good at by the end as I would make several passes through the paper going for the stuff I found harder and harder on each pass. The first pass would really just be a skim read and quick answer of the stuff I found easy, the last pass would probably be just going for one question that bugged the hell out of me.
I also never made any secret of this as my answer books generally had all the questions in the order I answered them (eg, 5,2,1,8,etc) so it kept the lecturers on their toes. This was perfectly allowed so why bother trying to leave space to answer questions you are skipping on that pass when you don't need to, however easy it would be.
The thing is though techniques like this are always a bit of a cop out. Persisting with something you find incredibly hard to the bitter end gives a real sense of satisfaction when you get it right, even if it is only worth half a percent. This is what university should be about: Satisfaction and learning for learning's sake.