How much does the writing sample matter?
There are varying opinions on the importance of the writing sample but most Admissions people agree that 1) it is not as important as the rest of the exam, 2) it is important enough to be read by Admissions, 3) it is important enough to take seriously.
1) You should not (and WILL not, if you enroll in a course) dedicate equal time to the writing sample in your studies. Students should definitely take at least one timed, practice writing sample. For those of us who are unclear about what the writing sample entails, it involves a scenario with two choices. You will choose one and argue for that position. Neither choice is right or wrong. You are being measured on how strongly you can argue a position.
2) Writing samples are not scored; however, they are often read by Admissions staff. They are looking to see whether you can coherently argue for a position. They may also compare your writing sample to the writing sample in your application to see how different they are (they want to see if you really can write or if you just have good editors).
3) It is unlikely that your writing sample, if taken seriously, will be the determining factor of whether or not you are admitted. However, if not taken seriously, you may be in trouble. If you choose not to do the writing sample, a BLANK page will be sent to Admissions of every school to which you apply. One Admissions Director told our Campus Prep class that a student thought it would be funny to insert a line in the middle of the sample that read "I know you are not really reading this." This is unwise. Several years later, she still remembers this and I promise that her reaction was not the same as the student's while writing it.