Quatar is a country with no crude oil in the ground, just natural gas, so they have a big interest in this. Only if crude oil stays at todays high prices and goes higher, will GTL be competitive for fuels.
GTL is about a lot more than just diesel fuel. You can build up any paraffinic hydrocarbon molecule using this process. Synthetic motor oil may cost a fraction of what it does today using GTL techlology.
In the US, methanol is the clean-burning fuel made from natural gas. Some diesel buses in LA now run on methanol.