I've always assumed it was due to closer shifter tolerances. If the wheel is on only one location you can design a more precese shifer. But as I said I am assuming. I kinda wish my road bike had them today when my wheel kept slipping 13 miles from home.
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I'm not one for fawning over bicycles, but I do believe that our bikes communicate with us, and what this bike is saying is, "You're an idiot." BikeSnobNYC