Wow, really? I have never heard of a school looking at PS before DAT. Thats pretty surprising. I think the majority of schools look at GPA and DAT first and second, just some schools look at GPA first and other DAT first and if you go to predents.com, youll definitely figure them out.
To your surprise you PS is extremely important. The people who are reviewing your application don't know you. That essay is the means by which you present yourself. They might look at your DAT scores and see that they're average, but then they might look through your PS and think "WOW! this person has had such unique experiences in life and he/she has such an interesting personality! So, let's invite him/her for an interview and meet him/her." This is where a person with an amazing PS gets chosen over someone with a not so amazing PS.
Also, many schools may consider GPA/DAT scores to make the initial cut, but when you make it through that things get more complicated than just scores and stats. For example, last year I was speaking with a professor at the UCLA School of Dentistry, who was on the admission committee before. He told me that they have a "formula" where they plug in your DAT, GPA, Score assigned to your ECs, etc. and generate an overall score for each applicant. It is that score (rather than your individual DAT/GPA) that determines whether or not you'll be getting an interview.
Also very important is the fact that your interview can break it or make it for you. No matter how amazing your stats are, if you don't impress those people as an individual, it's unlikely they'll take you.
So, many of us predents want to pretend like we know so much about the admission process, but that's not the case.
The reality is that lots and lots of various factors are taken into consideration before an applicant is even granted an interview. Also, chance plays an important role in the process. The selection process is done by people, who naturally have different ways of judging an applicant. So, one adcom may review your application and become very impressed, while another adcom might review it and consider it mediocre. Thus, our best bet is to do our best, submit early, and then sit back and wait for interviews and final decisions.