Every piece of your application is important and the secondary is no exception. With that being said, it is difficult to determine whether the secondary or primary is more important. An admissions officer may not be impressed by your personal statement but decide to give you interview because he or she really liked your secondary essay responses. On the flip side, your secondary responses could be bland but you might get an interview based off your accomplishments and personal statement. The secondary is often also known as a supplementary application. It is supposed to complement or add onto your primary.
One thing to keep in mind, however, is that every school sees the same primary application while you submit separate secondary applications to each individual school. That means, technically, it is not as bad if you mess up on a secondary application.