You actually make some great points, and I agree with a lot of them. There is SO much redundancy in the system. There are some counterpoints though:
1. If you got out of high school and did a 5 year program, you would be 23 when you became a "dentist". Now, that may not seem all to significant to most, but you really do mature a lot from ages 18-25. I can see it in myself even as I am about to graduate- I see a lot of immature activities even from my own classmates, really stupid stuff. Sure, they are fun to hang out with, and every field will have its clowns, but I look back and think, "damn, I sure wouldn't want this person treating my teeth". It's somewhat sad, but the maturity you gain in college and during these prime years really does make a difference.
2. Lots of people don't know what they want to do when they get out of high school. I know I didn't, but if you let someone on a 5 year path and then at year 3 (age 21) they decide they want to change, your dropout rates will be MUCH higher than they are now, and many people will have a useless partial degree and lots of debt. Think of how many people quit of drop out of college- it would surely be as high if this system was in place.
I suppose it's sort of a necessary evil, and in hindsight, its probably a good idea overall. It's tough to explain, but when you realize you are taking care of people's welfare, actually injecting them and drilling on them, you may appreciate it much more.
In response to another post- OMFS IS practically a "medical specialty". They pretty much reign supreme in the dental field, and no one has more training than OMFS in trauma AND sedation (minus anesthesiologists). 4 yrs college + 4 yrs dental school + 4-6 years of OMFS training make them as absolutely qualified as any physician.