The FLOPS/watt ratio might be higher than regular servers or desktop machines (might be, its not clear), but for a lot of small scale homebrewed deployments it'

The FLOPS/watt ratio might be higher than regular servers or desktop machines (might be, its not clear), but for a lot of small scale homebrewed deployments it's not realistic to expect linear scaling (given the problems/algorithms/available amount of development time/expertise) - in practice decent single-core performance is important. Ie. in many cases if you have the development and ops expertise to get stuff to scale to many cores, then you probably have the budget to get more serious hardware.