I agree that, in many cases, filling in behind the existing hairline makes sense for a lot of people. I suppose that the native hairs in front will naturally recede back into the transplanted hairline.
I have what Matt at the Shapiro Group described as a "three-finger hairline." My hairline definitely starts below the natural curve on my head, so he felt that it makes sense to start it behind -- which will hopefully preserve some donor and shrink the overall area that may need to be covered over the years.
Conversely, Dr. Sharon Keene -- who has done extensive research on hairlines and hairline placement -- pointed out that it may make perfect sense to fill in the hairline where it is today.
I guess it's best to collaborate with your surgeon to get the best results over a lifetime.