If you think you know better than Canon, Neuro, and all the other great people who give up their time and expertise on this forum for free, then fair enough. Get a petal hood, and enjoy watching it spinning around when you focus.
If you do feel like reading on, a petal hood is shaped like it is because the lens produces a circular image, but the sensor sees a rectangular (3:2) image. The lens hood is designed to shield the lens from stray light which would only add flare. The lens hood on a zoom is always optimised for the widest setting, so as to avoid shielding the corners/edges, hence the reason why the 18-55 needs a shorter lens hood than the 55-250.
The petal shape comes about because of the fact that more expensive lenses have front elements which do not rotate. Because of that, they can have a lens hood which is shaped to provide shading to match that rectangular shape of the sensor, optimising the shading. These petal shapes are exclusively found on lenses with front elements which do not rotate, and almost only on wider angle lenses.
If you find a petal shaped hood which offers the perfect amount of shading for your lens, the moment your lens refocuses, that 3:2 rectangle of shading provided by the hood will rotate, and won't line up with the sensor. There will be completely black corners or edges. It will be terrible. But if you find one like that eBay one you pointed out, it's the normal (shallow) one with badly and pointlessly cut out corners, which only reduce the effectiveness of the hood, and will wind you up every time the lens refocuses.
It's your choice, your money, and if you're certain you know better than Canon, feel free to get something off eBay which is made by people willing to exploit your viewpoint.