I think someone with a very low GPA (<3.0) will need at least 1.5 -2 yrs minimum of post bacc (3.7-4.0 A-/A) to show a trend (Unless the last 2 yrs of undergrad were much better than the first).
Those with a GPA greater than 3.0 might be able to pull it up to a reasonable number with a solid run of As (1-1.5yrs).
I personally, did not consider dentistry as a job until I was 27. Maybe some people don't realize it until they are 20, 22, 25, etc.
I had a very low undergrad GPA 2.3 (C+). I did not realize, at that time, I would need straight As to do what I would want to end up doing for a career. So, when I was 18-22, I worked more hours while in school, socialized more, and got the grades I needed to get the BS Biology degree.
I think dental schools give this option to people who dicovered dentistry later in life.
If you knew you wanted to be a dentist when you were 18, did all the pre-dental stuff (knew the expectations of DS) when you were an undergrad, yet couldn't make the grades ... its harder for them to see how you might could turn it around and be able to handle DS. (maturity and drive can change though)
I think the post-bacc is a route to show ability (how you have changed study habits) .... along with a solid DAT score. If you can show a better work ethic (ie. - better GPA-post-bacc) and a solid DAT (a reason for them to think you might be brighter than other traditional students) .... then I think you should have a chance.
Also, I think you need a strong foundation in intro BIOL/Chem/Org CHEM to really grasp higher chem/microbiol/biochem/anatomy/physiology classes. I think the post-bacc route, maybe, lays that foundation better than a masters (which might be better for a recent grad with fairly good grades). I think it would be very difficult to excel at higher level science classes without a solid understanding of the fundamentals.