To be clear, with regard to this product I'm not as concerned with objects hitting the occupants, I'm thinking about occupants hitting hard objects.
I'm reminded of a police officer that got into what would have been a pretty minor wreck in a parking garage.. except his head hit the sharp metal corner of the aftermarket emergency light control console that was roughly where our dome light controls are. The inside of his car and the area around it no s*** looked like a tarantino film. Scalp bleeds can be wild and he wasn't a small person.. I'd be surprised if he didn't lose a pint or two.
Yes, cargo within the vehicle is a concern. But the things bolted into the vehicle itself, if too close to the occupants, can be a serious hazard. From the factory all of this stuff is tested extensively, resulting in carefully managed geometry and surface textures/materials that lower the risk to occupants in different types of accidents. The fact that some budget vehicles have hard plastic (aka cheap) door cards in the rear seat only is not an accident.. that's what the regulations allow in that position. Sticking a metal frame directly above the heads of the people in the front row is asking for trouble, IMO. Not only through contact with the head, but arms, hands, etc.
For stuff not directly in the front row.. the backs of the second row seats will go a long way toward protecting occupants from flying objects in a frontal collision, where sheer energy is typically highest. Without being an engineer one exception to this would be interior-mounted spare tires.. my gut says that's just too much mass with it's center too high up to be sure it won't make it to the second or front row occupants.. even if it means folding seats over on the way. Other accident types add all kinds of unpredictable forces, but those forces are usually much lower, to the point that good tie-down practices can really make a difference. The problem we have then is the stock rear cargo tie-downs only use M6 hardware.. but even those failing would dissipate energy in an accident.
Also, no hate. People can take the risks they deem appropriate.. I just think many people don't really understand the risks. That's why I'm speaking up.