To me, Furious Slash feels like an ability that was throw into the spec to appease the people who just can't stand to have one second of downtime in their rotation.
The button-mashers, who claim a spec feels "too slow" if they don't have something to press every single GCD.
At the end of the day, Furious Slash just ends up feeling like Wild Strike. It even has the same 'wet fart' sound effect.