On every storage box with a "seat" that I've seen there are warnings in the literature not to use it for a passenger. I have seen plenty of people on 1-up machines with a passenger. Every one of them was squatting pretty drastically including my son's Renegade when he gave some friends a ride on the rear rack.

 

Even when you have no passenger the seats have pretty good storage room under the arm rests and back rest. Another nice feature is the foot pegs for the passenger.

They also act as rear fender protectors as well. When I sold the seat I kept the foot reast.

 

Thats the whole idea, they were meant to be used for storage, not for hauling a passenger. But as I said before, for leisurely, easy rides a passenger is ok. Don't try to take a passenger down the Rubicon trail or something similar. Unless that passenger doesn't mind walking most of the time. And make sure that passenger has some muscles too so she can spot you in those diffcult sections.