I "only" have three kids, so I envision using the rear seats for short haul trips when we're carpooling, taking nieces and nephews (or other adults) to a restaurant or game or something. For a long road trip, I'd put all three in the back seat and use the trunk for storage. This is what we do now, even with my wife's 3-row SUV. The third row only gets used when there are extra people and/or there is fighting or something between the kids, but that's pretty rare.
I just don't imagine sitting back there, after the novelty wears off, being fun at all to be not facing the rest of the passengers and riding backwards for more than thirty minutes.
BMW has those awesome automatic sun screens on all the windows, but I'm definitely assuming I would need to buy the standard kids screens for back there, even for short trips. I live in Florida, and even with tinted windows it will be way to sunny for anyone to be back there with no way to block the sunlight, even on a short trip.