Every word counts, both good and bad. Not only are you answering the question as only you could, but you are also demonstrating writing, grammar, and spelling skills, as well as being able to communicate in an organized, well-thought-out manner. Write your own essay, but have it reviewed, by several people. One for grammar (think English teacher), another perspective for "Did you answer the question?", and yet another for fluff and wasted words. The essay is an important part of the app. With respect to the "optional" essay, there is no such thing! Use every opportunity you are given to make your case.
A question that comes up every year is the character count (or word count for other essays). These are hard numbers, there are no exceptions.