A well-written essay is one that is well-supported by close textual analysis. According to our teacher, the highest mark an essay without analysis can get is around 15-17. (don't quote me on this...)
Unless the person's written expression is so poor, that they are unable to formulate an articulate response, their mark should not be affected. Most people say simple language is better than sophisticated language, simply because it's easier to read and understand. At the end of the day, what matters most is your ability to get your ideas across to the marker.
(I think... Idk)