The one thing you guys might be forgetting is that when you have two car seats in the second row, going to the third row from between the second row seats might not be possible. You would often have to either squeeze by one of the car seats or move one of the front seats way forward in order to get by. I have had to do this many times in a minivan and it's a pain!
I think that was part of the thinking of being able to automatically move the second seats forward even if car seats are already installed. They showed this very clearly in the model x launch. Access to the third row is easier this way and plus, you don't need to squeeze yourself by the car seats in the second row.
One of my reasons for going with 7 seats is that the middle seat in the second row is known to be one of the safer seats so I know I would put my newborn there as opposed to an outside seat. I did also debate this 6 seat or 7 seat preference and while I know it would be a lot nicer to be able to carry longer items like fishing rods if there was no middle second seat, I could not justify that knowing my little one is safest while sitting in the middle.
One last reason is if I have 5 passengers and a lot of gear, I can put 3 people in the second row seats and leave the third row and cargo area for luggage and any other gear I might have.