That's not necessarily true. The average PhD is about 5.8 years (lets round that to 6). If a PhD is there for 8 years, then most likely, they weren't a star PhD student...
Here's the break down:
A PhD student has about one year of class and about a year and half of rotations. During the second year, they pick their thesis mentor and then they have some pretty big teaching responsibilities. In reality, a PhD student doesn't really get into their thesis until 2 years in so they really only have 4 solid years of thesis work.
For an MD/PhD:
In most programs, you've done your rotations and most of your coursework (department depending) by the time you're done with MS2 (sure you may have a few grad classes to catch up on but that may only be for a semester, some may need another rotation, but that's an individual's preference). So by the time you start your PhD portion in third year, you're pretty much ready to delve into your thesis work. At most programs, you have about 4 years at this point (3 if you're good).
So in the end, you're doing thesis work for the same amount of time as a regular PhD. It's true that the PhD student may have a semester or two on you, but MD/PhD students tend to be more focused and accomplished (with a very strong research background) than your average PhD student (as PI's have told me throughout the interview process).
Now there are true differences in the programs that give PhD students an edge. Phuds go right into their post-doc with no break from their thesis work so you could argue that they're better prepared to get into it. Also, PhDs are allowed much more leeway in their thesis work to try things out and really take on 'risk' projects. Risk projects take longer to do, but if they work they're normally more of a breakthrough than your average safe project. However, in many cases, these risk projects don't work out and the PhD student is put on a safe project in the end to graduate them.
MD/PhDs also have an advantage. They have first hand knowledge of the challenges and limitations of medicine. They can put a disease in perspective and have a better understanding of what needs to be done in the realm of basic research. Ultimately, an MD/PhD will ask different questions than a PhD and there's nothing inherently better or worse, they're just different and you have to decide what you think is right.
Not to mention that you can have clinical responsibilities on the side that can enhance your research (and salary). In fact, here at the NIH, there's a PI who's an MD/PhD and he doesn't have any clinical responsibilities. He gets paid WAY much more than his PhD counterparts for doing the exact same work. I'm not sure how it works in academia, but having the MD definitely makes you a more attractive hire, especially in academic medicine. Not to say the safety/money should be a goal, but it's an important thing to consider in a world where funding is getting tighter and tighter and there are more and more PhD's struggling to find positions. The MD will set you apart in any situation whether in academia or industry. A degree can take you a looong way.
There's something else that an MD/PhD can get you and that's what I like to call 'career insurance.' My boss is an MD who came to the states to do research for a year. If it didn't work out, it was no problem and he could just go back to his country and have a very successful career as a clinician. Since he had this 'insurance' on his career, he was able to take more risks than he probably would if his life depended solely on research. As I said before, the risk projects have a high chance of failure, but if they work, you're golden. So, he did a risk project, it worked out in the end and he's been an incredibly successful scientist his entire life. Now, I'm not naive and I know this could go both ways. Having this career insurance can make someone a bit lazier. Why bother really pushing myself in research when I could just go into the clinic? But this is something that has to be decided on a personal level.
So that's my big rant about the differences between a PhD and an MD/PhD, but really, I'm not going to pretend to know anything because I really can't know until I make my decision and I'm done with my training. I'm actually at the verge of choosing between an MD/PhD program and a PhD program and this was kind of a way to spell out my thinking process and how I'm going to make my decision.
As of now, I think I'd choose the MD/PhD path and I have an acceptance at a great school (and there could be more to come), but I also have PhD acceptances at some really great research institutes (Harvard, Berkeley, MIT (possibly)). Those are what I see as the pros and cons of both sides. In the end, I'm a researcher at heart so we'll see what happens (it also depends on where the significant other gets in).
