The admissions office tends to use the tragic accident test. If the applicant suffered a tragic accident in the summer before he/she was to start at MIT and would never play their sport again, would MIT still be the right university for that applicant?
If the answer to that question is yes, then the letter from the coach to the admissions office can be very helpful in the admissions decision. If the answer to that question is no, then there is nothing that any coach can do.
Keep in mind that the dean of admissions was once an MIT varsity coach. Stu Schmill really understands the process from both sides.