GPA is the indication of a person's ability to survive or even excel in dental schools. DAT is an indication of seeing if you can do well on the boards by putting all the info you've learned in school and put them to good use on a stand. test.
There's a cut off pt because obviously schools wouldn't pick students who can't handle undergrad with a 2.5 over someone who's a 3.8 right? Also keep in mind that some people have to balance between working full time, going to school full time, and also trying to beef up their resume so that'll sometimes kill a person's GPA until he/she finds a good balance between all those areas.
Yes, some people might have like a 3.9 GPA...good, that means they can probably do REALLY well in dental school, didactically. However, can he pass his pre-clinical courses? People with 3.0.......ok maybe they won't be #1 of their class but they can still graduate and become an excellent clinicians, right? Don't forget, being a good dentist is not about book smarts, it's about being personable, leadership, having good communicating skills, being responsible, and having good hand skills. Maybe a person with low GPA didn't do that well of a 3.5 in school, but that doesn't mean that applicant won't do better than the person with a 3.5 in undergrad IN LIFE.