The three other essays in your application + essays in your secondary will be used to see how you write. The writing section will not be the difference between you and another applicant.
I review several hundred MD-PhD applications a year, and the only time I note a writing sample score is when the value is "J", which falls in the 1.0 percentile of MCAT scores. Typically a "J" correlates with low scores on other sections of the MCAT.
I would say a really low score will hurt ... a middle or excellent are negligible.
Also, I always ask my friends (who ask about MCAT score advice) to break down B/P/V ... as the B/P parts are more important than the V in general as well.
