The thing I question is that, are there a low number of non degree students because more prefer to get the degree and do everything over a 4 year span? I don't see why it would be harder as long as you have all the pre reqs and 90 credits, because what use will your degree be of when in dental school?
The way I've come to terms of adding value to my undergraduate experience is that the course-loads teach students how to adapt different study skills and manage time. Science courses, in general, and research, specifically, teaches you to (1) be critical in accepting statements as truths, (2) apply logic and reasoning, and (3) be open-minded and curious yet always skeptical.
If we weren't able to find these worthwhile investments from spending 4-years in college then we ought to earn Bachelor's of Dental Science immediately after high school like in some other places in the world.
The degree itself is a pointless credential. I mean you could have not completed an Arts course but have completed everything else and not be awarded the degree. Completing the degree just makes the school look better because the matriculates have been credentialized (I made up a word).
Applying within three years is hard. A typical applicant would have to take General Biology 1&2 or just Cell Biology, General Chemistry 1&2, English, and Calculus by the end of the first year. At the summer between the first year and second year, you would have to take as many prereq courses as possible. During the second year, you would take Organic Chemistry 1&2 and Physics 1&2 (upper-level English and maybe Sociology and Psychology). Before the end of your second year, you will have already racked up hundreds of hours of volunteering and shadowing, have asked for letters of recommendation from professors and dentists, and have written your personal statement. At the end of your second year, you will study for the DAT (most people study for a dedicated month) in the summer and simultaneously apply to dental schools. During the late summer to early Fall of your third year, you will write supplementary/secondary essays, go on interviews, and stress some more. During the third year, you take Biochemistry (and Physics instead of taking it during the summer) and as many upper-level courses as possible while stressing about interviews. By Dec. 1st, you'll hear back from the earliest possible acceptance.
Basically, the admissions committee will only have two years of mostly non-upper level (bare minimum) course work to evaluate you by compared to normal people who will have three years, usually with plenty of upper-level courses during that third year. Unless you have straight A's, the admission committee's sentiment will be "Why accept this kid instead of someone who will have completed everything plus more and earned a degree?" In terms of competitiveness, you will always be at a disadvantage when compared to people who did just as well as you on the prerequisites but have completed more upper-level and harder courses and have racked up more hours of research, volunteering, and shadowing. The school doesn't gain anything if they accept an average applicant who applied during their Sophomore year. You have to prove that you're worth it.
Applying a year early is stressful because you have to have everything ready in a very short time.