Lens hoods are like coat hangers; they multiply when left in isolation. Actually, there’s a reason for that big pile of lens hoods in your camera cabinet: it’s not obvious which ones go on which lenses. A new lens comes with a lens hood, but you don’t always use it with the hood. When you get rid of the lens, it’s too much trouble to find the hood that goes with it, so there’s one more orphan hood in the pile. That makes the pile bigger, and thus makes it harder to find the hood for any given lens. And so on.
Reversible hoods make a difference; you’re more likely to use the hood, and less likely to let it get separated from the lens that it fits. However, they’re sometimes awkward, and they get put in the pile, and we know how that ends.
Here’s a plea to the folks at Nikon. In addition to putting HB-25 or HB-679.34 on your lens hoods, put the name of the lens that that hood fits on there too. Hasselblad does it. Zeiss does it. You can, too.
On the other hand…
Nikon bayonet lens hoods can go on in two positions 180 degrees apart. Either orientation works fine. You can put them on by feel; just twist them until they drop into place, and keep twisting to lock them.
Zeiss bayonet lens hoods have their locking tabs 120 degrees apart. There’s only one right way to put on the hood, and you’d better look carefully when you mount them, because if you put them on the wrong way, you can damage them. A replacement for the 21mm f/2.8 hood costs more than $200.
Nikon lens hoods are made of cheap, but durable, plastic. If you drop one, it will probably bounce and be no worse for he wear. Zeiss bayonet lens hoods are made of substantial-feeling metal. They ooze quality. If you drop one, it will dent, and may not work anymore. Did I mention that a replacement can cost more than $200?