Ensure your Cover Letter is Written is Very important as it shows that you can write a sentence and allows you to get important information into your Resume that you will need for your application.
Writing a Cover Letter is both important and difficult as its the first thing that will be read by a hiring manager and therefore the first impression that you will give. Get this wrong and your resume will be in the bin.
Some people say "yes", and some say "no way" but there is one rule that says "if you're going to do something, do it properly or don't bother doing it in the first place". A good cover letter will always be read.