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I mean, really it doesn't make any sense. Autarchs are the master captains, the ultimate Exarchs, the ace in the hole of Eldar warriors. They are masters of many paths, and they excel at using the weapons and wargear of all aspect shrines.
It doesn't make sense to me that they can't choose at least one, if not two, exarch powers for a points cost as well. Being the masters, they should know them just as well as the lower level Exarch.
Not to mention, it would make them way more fun, interesting and playable in my opinion. Some Crack Shot, Defend, Withdraw, and Acrobatic would be most fitting for your supreme army commander.
I think the reason honestly isn't in the fluff, but that they didn't want to make it legal to basically buy Exarch powers for aspect squads that aren't supposed to have them. IE scorpions with an Autarch and Defend.
I believe that is the exact reason. It wouldnt make sense for scorpions to have acrobatic as their armour is much too bulky to "dance around and shake their booties" like the banshees could.
Also the power shadow stike doesnt confer unto the exarch if he joins the squad, thus having a double negative both ways with Autarch having powers and not having powers the squad has.
I agree though. The autarch just cant compete with farseers powers to reroll dice. Whic is what wins eldar games.
Eldar -APOC- 6500 Points
Chaos Daemons -Fate Crusher- 3500 Points
Grey knights - 1000 Points
Most of them are supposed to affect the whole squad, if the Autrauch joins, I don't believe he'd confer those abilities. Some WOULD be useful (like Fast Shot, Crack Shot, Tank Hunter, Skyleap, etc...)
Exarchs are aspect warriors that get locked in the path unable to ever change paths again. An autarch is a leader that has experienced all the paths and moved on into the path of command. To develop the extreme skill of an exarch one has to become an exarch and thus become unfit to command the eldar in battle.
I would like to see some of the exarch wargear as options but the powers wouldn't fit fluff.
Autarchs are not 'masters'. The Exarchs are the masters.
If anything, the Exarchs and the Autarchs have the same amount of training. The Exarchs focus on a single path whereas the autarchs focus on multiples. The Autarch's desire to go multiple paths allows them to be better suited for leadership.
I believe the answer is "they are not exarchs" and have never been exarchs. I thought the premise was that the Autarchs had trodden the paths of the aspects but never got trapped by them (thus never become an exarch), in other words an autarch may have spent time as a fire dragon or a scorpion, but they have never been a dragon or scorpion exarch.
Its probably the same reason why they can take aspect warrior wargear but not exarch wargear, as much as I would love to give my autarch an executioner or star lance.