Hmm... if Toyota intended for their to be a 3rd row back there, I think there would be one. I would be concerned if I got rearended with folks sitting that close to the back of the vehicle. Are there crumple zones back there that would compromise their safety? How well do they mount back there that it would be safe to actually tether a child/baby seat to it? Then of course, the visibility is so good out the back of the truck... why not stick a few more heads in the way
That being said, it would be good for certain occasions that I want to take more people with me... just glad I wouldn't be the one climbing back there