The DAT is only a test. Sure, it is standardized, but your argument is flawed because you are assuming that everyone will perform up to ability on a standardized exam. I am sure that you know quite a few people around you that are smart but are not good "test-takers." If the DAT were the end all-be all of all gauges why not just have that as the sole measure and abolish GPA from consideration?
You also neglect a very important consideration. Taking the DAT and doing well on it is only a 4.5 hour endeavor, comparatively less grinding than the demands of a full, four-year curriculum in a good university. Some folks may be good test takers, but ask them to do consistently well across a breadth of classes is a totally different story.
It DOES take a lot of tenacity and courage to go through a demanding, tough academic curriculum. Remember that Dental School is not an easy experience--you have people that are the selected cream of the universities competing with you. Having this kind of experience in college will better acclimate you to the tough and challenging atmosphere. While I certainly admire folks that show tenacity in other aspects of life, we are discussing academic capabilities here, so let's not get carried away.
When I was an undergrad and a grad at Rice and Berkeley, respectively, I served on the admissions committee as a student representative. Without exception, whenever we sit down and review an applicant's file, we always pay particular attention to the institution/school where that student was from, because that gives us a idea of just how valid his/her grades are. Why else do you think the DS ADCOMs recommend no more than ~40-60 hours in a CC? If the numerical GPAs were all that mattered, then why go to a challenging pressure-cooker like Rice when you can get a 4.0 with a fraction of effort at a JC or CC?