Also because most designs rely on pressure rods to hold them rather than being bolted to the car they could come loose if the fittings had been improperly instaled or rattled loose. Seat belt harness do a good job of securing the dog but they aren't very user friendly. Many are difficult to put on and take off. Because of this it is easy to get lazy about using them every time you put you dog in the car. Also in order to be effective the connector between the harnes and the seatbelt needs to be quite short.
Some dogs have trouble adjusting to being tied that short.
Crates also do a good job secureing the dog. In and accident the dog will be thrown aginst the side of their crate but most dogs can be thrown a foot or so aginst a flat surface and come away with only bruses. The two door wire ones that you mentioned are my personal faviorate. The main draw backs of crates is that they are heavy and bulky if you want to move them and haul something else in your cargo space. If I had the space for one I would use a crate.