Less weight on the rear wheel, no rear derailleur that adjusts or comes off, and the weight is central and low: there are some good reasons for saying goodbye to conventional gears in favor of a transmission. This seems to be in the pipeline at Atherton Bikes. 

One of the motivating factors for building a geared bike could also be the so-called 100K Belted-Purse-Challenge. Belt manufacturer Gates is paying out 100,000 euros as a prize for the first World Cup downhill victory on a bike with a belt.