You bring up a very good point.
However, lets consider a person who slacks off completely through college, gets a ~2.8-3.0, and graduates without learning much in terms of sciences and such. If all this person needs is 3 months of preparation through Kaplan to compensate for 2 years of undergraduate work to score really high on the DATs, then this person is a genius and should do pretty damn well on the NDBE and any school (especially Columbia, UoP) would be happy to take them.
You are correct. Which brings up the point that folks have to go through a subjective door as well as an objective door in order to get accepted into dental school. Folks way at the bottom end (<2.0 GPA & < 13 DAT) will never get past the objective door. However, even the folks at the top end (>3.8 GPA & >26 DAT )will have to get past the subjective door. Of course, every thing else being equal, the higher the stats, the greater the chances of getting through both doors.
Bottom line, like you imply, is that some folks who would make great dentists may never get into dental school while others who get into dental school prove to be real disasters for the profession (read some of the states' dental licensing board investigative reports). It is not a perfect world. So the operative rule for those wanting to go to dental school is "do all you can to present a quality package to the adcoms and keep your fingers crossed."