The most useless accessory I have is the turbo-nozzle that my miele came with (as seen in my first Tristar vs. Miele video), the brushroll is loose, by design, so that it can bounce up off the surface it's meant to be cleaning, so barely sweeps carpeting to any effect, and has soft bristles, so even if it does sweep, it doesn't really do much, all it needs is a couple of springs with enough tension to push the brushroll out to keep it in contact and to keep it stable, but, nope, they omitted that idea and the thing just sits in a box, unused and unwanted...
And one other accessory, that little clip-on brush that goes onto Kirby crevice tools, doesn't do anything for me, gets in the way, and often falls off, I understand the idea of it, but, nah, doesn't win my vote...