To Exarch or not

The question is, should you take an Exarch in your aspect warriors?

This is actually more of a difficult question than it seems.  Not all Aspect warriors benefit as much from their Exarch as others.

The key is to look at the skills the bring along in the form of warrior powers.  Some Exarchs are nailed on essential in even a 5 man squad, while others are only useful in bigger units.   Remember Exarchs are your Sgts and Special weapons guys rolled into 1.  

In my opinion Striking Scorpions, Howling Banshees and Shining Spears should always, always include an exarch, even in minimum sized squads.  The reason for this is that Exarchs have strictly superior weapons and statistics to their squad, and the cost to upgrade them is more than reasonable considering you get better WS and BS as well as access to much more powerful close combat weapons.  Always take Scorpions Claw, Executioner and Star Lances.

Swooping Hawks really don't get much mileage out of their Exarch.  Their special guns really are not all that great, and neither are their abilities.  I suppose you can attempt to blind someone, but I would rather just field 6 for the large blast and leave it there.

Warp Spiders maybe want an Exarch, but only if you want to use him with the Spinneret Rifle and maybe Power Blades.  Against certain armies he turns the unit into a reasonable melee threat.

Crimson Hunters should always be Exarchs.  Better BS, can snipe characters, weapon options.  All very worthwhile.

Fire Dragons it depends.  I always take one with a Firepike with fast shot, as his damage output is greater than  two normal dragons this way.  The Heavy Flamer wastes his exceptional BS to give some decent anti infantry firepower, but I suppose he can man a quad gun or something.  That said, the unit doing its primary objective of wasting tanks, don't actually need an Exarch.

Dark Reapers.  Again it depends, as a small squad of three is quite lethal vs marines and the Exarch pays all kinds of extortionate taxes on his upgrades. But the Exarch adds flexibility. With my favourite heavy weapon in the entire game in the Tempest Launcher, I always include him.  Fast shot is a must.

Dire Avengers.  Leaving the worst until last.  Their Exarch varies between being completely pointless and quite decent.  I would only ever include him if you have already got a full sized squad of Avengers, and then I would always give him the Power Weapon and Shimmer Shield combo to boost the entire unit.  He really doesn't have the statistics to go around assassinating multiple wound characters with a Diresword, a twin linked catapult is laughable compared to what it was in 4th edition and sword and pistol really doesn't make the squad a valid combat threat.

Now you might be wondering why I say the power blade option on the Warp Spider Exarch is a good idea, but the power weapon pistol combo on the Dire Avenger isn't.  The reason is the Warp spider keeps his considerably decent gun, and doesn't have a pistol.  He is also more liable to survive combat due to his better speed, and has hit and run.