It's all fine and well to use chengyu, but that being said, it isn't necessary to do so to get a high mark and in some cases it can even lower one's mark. Many students struggle to use memorise chengyu, and even more struggle to apply it appropriately. For those students it is far preferable to master the use of relatively simple language and apply it well. After all, so long as your essay is logical, addresses the topic and is grammatically correct no one can score it less for its lack of chengyu. When students cram chengyu, they often fail to remember them correctly, or they apply them in the wrong context or they simply complicate something which needn't be complicated. So while it's good to learn some universal chengyu, I think in this case quality vs quantity applies, and quality can be achieved with little chengyu.