I think the line BMW gives is more to protect themselves legally. They can't recommend full use of the 3rd row and risk an accident killing a baby etc.

My dealer highly recommended I not bother getting the 3rd row and I have regretted it every day since picking up the car. I have a friend with the x and uses the 3rd row all the time to pick up and transport kids. It does its job. Not meant to be used for 6 ft adult on a 5 hour road trip. Understanding this, since I don't mind the reduced storage and loss of spare tire, I've regretted not having it as I could have used it several times in the past few weeks. The following is my understanding from some research:

1) An average size adult can also sit in the 3rd row though it won't be a comfortable ride. You can adjust the 2nd row seats for additional room when you have 3rd row seats.
2) the 3rd row does not have latch, so if you really want to use a car seat, it will need to be with a seat belt. It is quite difficult to get an infant out as it is difficult to reach, but kids that can walk can help themselves out.
3) if you have a rear entertainment system, 3rd row seat occupants see virtually nothing since it does not drop down from the roof