I think you've misunderstood the concept of the Autarch though - as portrayed in the Swordwind supplement for Epic 40k and the recent Index Xenos article.
An Autarch isn't like an Exarch. Autarchs are consumed by war yes, but they step from one discipline to another, their entire lives devoted to war.
Exarchs are a *little* less diverse. They're trapped in a particular aspect - and that's all they see. Can you imagine a Howling Banshee leading an army?
***
"Right everyone, run at the enemy as fast as you can... and when you get there: Scream!"
"But miss" says the plucky Guardian "i only have a Shuriken Catapault, and i'm not much good at screaming".
"Silence you, i've been on this path for seven hundred years, and i know what i'm talking about."
***
Whereas an Autarch has trained under several different Aspects, served as a Guardian, perhaps pilotted Jetbikes or Tanks, maybe flown attack space craft. His/Her accumulated knowledge would make him/her far more valuable as a leader.
Unlike a Striking Scorpion, who would probably berate the Dark Reapers for not 'Sneaking enough'.
Meh,
Adam.