Actually its not. The reasons its not is becuase with the next few process shrink the power advantages will be a moot point and the software porting and creation costs will significantly outwiegh the minimal power hit.
when intel and AMD are at 28/22nm it'll be a moot point. Plus with new power saving features and better power gating, they will be able to be control clock and power consumption. Also integrating graphics and most other outboard chips onto one die will drastically reduce power consumption and massively improve perceptual performance.
its the software ecosystem and if the devices are x86 complaint then the x86 ecosystem will simply run on it provided the peripheral chips on the rest of the device are solid.
Your reading the costs of ARM in the wrong way. The cost of ARM isn;t the chip itself or performance/watt. Its about applications. If these devices are x86 then most current software from the desktop space will simply work.