Keep in mind the standard /normal way to haul 8' 2x4s is to lean them over the tailgate of a pickup truck, sometimes tied to something, sometimes not. I'll address some questions:
1) They'll fall out.
Compared to the pickup truck default, it aint happenin. They were actually wedged tight, and didn't so much as wiggle the entire 7 mile ride home. The bungees were a secondary/overkill securement. Try this at home. Feel how little room for movement there is. Explore what forces would have to occur to dislodge the boards. Compare this to leaning them over a pickup tailgate.
2) Put on roof?
Up there, there's nothing to tie to (nothing to tie to on the 2x4s themselves either). Also you don't have gravity or wedging effects working for you - it all depends on your ropes not stretching or shifting. I've had loads come partially loose on a roof rack, but never in the way shown.
3) Put through trunk?
I actually tried multiple approaches, including this. Putting the boards through the trunk/back seat would leave the trunk lid partly open or flapping up and down, would risk the boards interfering with the driver, and funnel carbon monoxide into the cabin (see owner's manual). For all those disadvantages, you'd still be dependent on the friction of your ropes or bungees to prevent the boards from sliding out the back and onto the street when going uphill.
4) Width
No one mentioned this, but I'll address it anyway. This technique widens the car to about the width if a dually pickup truck. So if duallys don't bother you, this shouldn't either. As with pulling a trailer, one must be aware of the dimensions of one's vehicle on turns, etc.
Overall, this is the difference between dangerous and unusual. Any better way to do something is unusual and scary, but to assess danger, you have to study the concept, identify the pros/cons and failure modes. This isn't just a funny picture, it's a technique that works. Don't argue on the internet about it, go out to the garage, find a board, and try it.