I've just been looking into them as well.Looking at the Gripsport tilty rack, what are the pro's and cons of the wheel hoops vs the 'tacos' ? mostly for 29er, 2.4" - 2.6" tyres
From what I could find looks like the tacos help with adjustment. As in, you don't need to adjust them as much to get the right fit.
With a hoop, you're stuck in the one spot, the tyre can sit anywhere along the length of the taco without needing adjustment, you just strap the wheel down with the included strap.