For belt drive it makes sense, but belts and pullies are not chains and sprockets. Adding a tensioner and a 3rd sprocket would increase weight and noise significantly and you'd have a 3rd sprocket to replace PLUS more wear on your chain. It would also probably cause tension issues if you went to a bigger back sprocket because the line of the chain would be at a different spot in the stroke of the tensioner arm...and chains don't need to be nearly as tight as belts because of the engagement depth of the sprockets, and chains need that slack to prevent wear. It would be better if all bikes were just belt drive...nice, quiet, clean, low maintenance belts...