Actually, the Superbones has brackets and straps that secure the down tube to prevent the bikes from swaying excessively (oops, just looked at the manual, they are intended to attach to the seat tube, oh well, they work great either way 😁). They work quite well to keep the bikes stable on the rack. You can see the straps in the picture below indicated by the arrows. The rack is very sturdy and doesn't shift at all even on bumpy roads with 3 bikes on it.If you look at the photo above, when you are driving down a road, the bumps and movement of the car with the wind against the bikes, they will sway back and forth grinding the paint off each other. As well the rear derailleur will get constantly bashed against the bike next to it and cause it to break. (Generally while you are riding).
You're right that everyone's requirements are different. For me the additional hassle and cost of mounting a hitch and then getting a hitch mounted rack far outweighed the benefits.