Just as a strong personal statement can get you in, a poor personal statement/essay can quickly eliminate you from the mix. When a medical school committee has a few hundred applicants, seeing typos, spelling mistakes or bad content will quickly move you to the bottom of the pile. The personal statement is a quick way to "weed out" similar applicants. With two similar applications, the personal statement can decide who gets an interview and who does not.
The personal statement is also a great place to explain more serious problems, like less_than_ideal grades or test scores. Don't be afraid to write something like, "I don't test well" in the statement. As long as you can provide a good reason for you app's problems, it's okay to address them directly. You won't be able to hide it from admissions officers anyway, so providing a reason for problems is always preferable to ignoring them.