Thats not a bad idea Chris. But I would imagine the backrest to be a bit more comfortable for the passenger. Now if the top box could have an integrated back rest on it too that would be accommodating.One option...is to buy a top box. That will give you a place to put things for you and your passenger, and provide a backrest as well.
It'll pay to think ahead on this. Many bikes don't have provision to install a backrest...and then install a top box behind that. You could end up spending the money for a backrest, then buying a top box and having the money for the back rest wasted.