I think it is both: a huge waste of time, but also worth your while.
It just depends on how your exam breaks down. I had 3 or 4 plant questions on my bio section. Missing 3 or 4 questions could bring your score down from a 22 to a 19. Of course, if you don't miss ANY other questions, then you'd probably end up with a 23 or higher if you only miss 3 or 4 plant questions.
How well do you know the other stuff? If I were you, I'd learn it by heart. You never know what your bio section will be comprised of. It would be tragic to find 10+ plant questions on your exam!