This is interesting to me - I have always tended to see myself as both a private practice guy as well as a academician so my take might be a little different than others'. As some said earlier in this thread, if a 6 year fits you then awesome, do it! However, I do also disagree with some of what has been said.
Disclaimer: I am a resident at a 4 year program, currently in my third year. My attendings are both 4 and 6 year guys, and a lot of the private practice guys that we interact with are also both. I know a lot of the 6 year guys who say it was a waste of time. Simply put. Understand that I am not saying it is, but I have definitely heard it, and heard it from more than a few people.
Here is my take: If you actively enjoy the education, then go for it - it is great education! If you have an ego and/or some inferiority complex and need the MD to feel better about yourself, go for it! If you want it just because you are curious about what it will give you, go for it! There is nothing wrong with getting the MD, nothing. Just understand the very real, guaranteed time and money costs. If you are OK with the costs, then enjoy what you are getting!
For those who say that they communicate better with other physicians - I have a hard time believing this one carries much value. At my program we spend a lot of time on anesthesia, trauma, SICU, ED, medicine, plastics, ENT, etc, and I am assuming most other programs do the same. Not only that, we are expected to work, act, and perform at the level of any other resident on those programs. We work our ass off to make sure we learn, learn fast, and work hard. We have earned their respect and are treated like colleagues. I have a hard time believing that doing med-student clerkships in these same areas would really change the way we communicate with these guys in any substantial way. Sure, you would have had a longer total exposure, but how much more benefit would it really have? In the end I am able to communicate with my physician colleagues comfortably and with confidence about our mutual patients. Furthermore, if someone goes into private practice, this will become an even more distant concern.
If you want to go into academics, then 4 vs 6 really has very little weight. You can easily do academics from either if that is your plan. There have been a lot of academic positions open at many different programs looking for people and many are having a hard time filling them. Also, a lot of people who go into fellowships and then into academics don't always operate in the area of their training. I have heard a number of people who are fellowship trained get frustrated because they don't have much of a case load and are still doing general OMFS procedures. Happens all the time. The fact is, if you are set on academics, you can get their going either route. Sure an MD might be a little easier, but two years and a few hundred thousand dollars easier? I'm not sure.
Anyway, sorry for the longer post, but I just felt that I wanted to give my anecdotal opinion haha