It's only a 'waste of time' if you either a) if you're an idiot for not considering the syllabus which has the key words to adhere to or/and b) do not know your texts in their entirety. That's why you have to make sure your generic essays cover all areas of the rubric to ensure flexibility so you can adapt to the question on the day (if it's a curveball, then say for instance, belonging, you can make links because obviously the AOS is heavily interrelated).
The problem with the bolded part is this - you may find that method of study works for you and if it does, then great. But the thing is, not everyone is adept with the perfect English skills. Everyone learns differently, and so you can’t expect everyone to be able to answer directly to a response. Some people can’t do this. There are people that aren’t natural writers. There are people all though they have tried to become fluent writers, are still yet to grasp the skills. Some people are naturally good at English. There is a giant imbalance in the English skills of everyone. I think the whole concept of examinations is rigged. Everyone has a different learning style. How can one accurately test the knowledge of one’s abilities if everyone thinks differently?