As many of us know, it's still possible in many situations if you can get the front door open just enough to squeeze through. The reason why you want to engineer something better for the rear door is for families, mainly. You need a bigger aperture to help kids in and out of car seats, for example. That's a huge use case for the Model X.
If you have kids and you watched this you are going bonkers right now.
Normal doors do just as bad in those conditions. Falcon doors perhaps block better at the side and worse, obviously since there is a hole in the roof, at the center.