I'm sure people do. And some may get away with it. I can't speak with certainty on the durability of every OEM crossbar. And I'm not against putting a tent on your crossbars to see what they feel like. There is value in satisfying your curiosity. On the other hand, Yakima and Thule are both roof rack vendors that got into the tent game. You may find something you like between one of them.
Another reason I'm a proponent of securely mounting a tent: back in 2020, a young driver ran a stop sign and intersected the front of my FJ on the passenger side, spinning me around at 35 mph and causing the vehicles to come into contact at least twice. Nothing on the roof rack came off. This is the same tent and awning I'm still using.
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The tent floor has 4 holes in the bottom where it bolts to its own aluminum crossbars. You can install them length-wise or width-wise on the tent floor to orient the opening of the tent to suit you. Every option has pros and cons. I've had mine oriented both ways. Opening to the back was easier to setup, but you had to deal with the ladder getting in the way. Opening to the side is harder to setup, but gives you some good shelter off the side of the vehicle.
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