The Review -
Dire Avengers are the most common Aspect Warrior and for all intents and purposes are Guardians with better guns and a better statline. An important distinction however is they lack heavy weapons which means by themselves, they are not good rear objective holders. That being said, in today's game Dire Avengers are basically used as a 'scoring upgrade' to a Wave Serpent or Falcon which is due in part to their cheapness (25 points cheaper than your cheapest Guardian squad) due to squad sizes of five and their complete lack of heavy or special weapons.
Beyond this though, Dire Avengers have the usual Eldar Aspect statline with good offensive stats but are weak defensively (T3/4+) though not so much as Guardians. The 4+ save means they won't die to your basic anti-infantry bolter fire but are still quite vulnerable to mass firepower or anything with AP4 if caught in the open. Importantly, they have BS4 along with an improved shuriken weapon which gives them a shooting threat range of 24". If this distance can be maintained for as long as possible, only units which can move 12" in any phase or have fleet can access the Dire Avengers after being shot by them. This is a serious improvement over Guardians and provides the Eldar player with more options in using Avengers but those options are still centred around anti-infantry, you can't make Dire Avengers deal with tanks.
The Exarch brings several options to the table though none of them are fantastic. Double catapults is cheap and gets you two extra BS5 shots so if you're grabbing the Exarch you should at least take this option. The Diresword + pistol is like the Powerfist on Tactical squads, except worse whilst the power weapon + shimmershield can make Dire Avengers an annoying tarpit when combined with Defend. However, this is only really effective against big beefy units which would normally carve through such things due to awesome attacks - this combination will do nothing to slow a mass of attacks from Orks, etc. Bladestorm is likewise lacklusture though has some application. Over the whole game, Bladestorming Avengers will shoot less than normal Avengers since they cannot shoot the next turn. Whilst this extra firepower in a single moment can be used effectively (i.e. ensuring a unit dies, nothing to do next turn, etc.) using it consistently is often less effective than simply shooting normally.
All of these options make the unit more expensive however and you're already paying a fair amount for what is an anti-infantry unit with a T3/4+ statline. They can provide some additional options but they also serve to make the unit more inefficient.
Potential Uses -
The most common use of Dire Avengers is as cheap squads acting as scoring upgrades to Wave Serpent or Falcon vehicles. At 60 points for five models, they are a pretty cheap way to make the already expensive Eldar vehicles more viable in the 5th edition Troop based environment. There are quite a few uses beyond this though but are less seen as you arguably dilute the advantages the Eldar army currently has going for it.
A simple extension of the MSU squads in transports is larger squads in transports. It doesn't cost much more (another 60 points for 10-man squads) and doubles the survivability and firepower of the unit - useful for adding in some anti-infantry shooting for an army which can be relying on S6 to do all the heavy lifting. Whether or not you take extra upgrades like the Exarch, Bladestorm and Defend is up to the individual but this generally detracts from their efficiency as discussed above - one can certainly get decent use out of Bladestorm (last turns, need to kill that unit NOW, jumping in and out) and Defend (tarpit expensive units with shimmershield) but they will never make the unit amazing.
Running them on foot is also seen in Footdar lists where their 18" range is superior to that of Guardians. Although they don't pack Heavy Weapons, being able to shoot your opponent without always being in assault range the next turn is a nice option to have. Whichever way you look at it, running larger Dire Avenger squads will greatly benefit from Farseer support much like the rest of the Eldar army.
Potential Changes -
Price drop! Dire Avengers are difficult here as they are nearly always going to be competing with Guardians in terms of general role - scoring + mass shuriken fire. Making Guardians more viable by running them with 18" shuriken guns means Avengers really need to be at least 24" and even then, having a heavy weapon platform + 18" guns might see Avengers tossed onto the midden heap. A points drop and perhaps a more regular firing Shuriken weapon? Assault 3 or something? They need to be able to do something above and beyond what your regular Guardians do and little tactics like tarpitting and bunny hopping (two units using bladestorm) isn't really enough but we don't want them to simply be Guardians with better statlines either so keeping the duality role for Guardians with heavy weapons (and potentially support platform teams or something) works well in that regards.
Being able to add a Warlock to Avenger squads would also open up some nice possibilities depending how they work whilst gaining some benefit from running Asurmen would see more usage of that Phoenix Lord and a way to improve your Avengers beyond your stock standard dakka scoring infantry. What though...?
Conclusion -
Dire Avengers feel the dating of the Eldar codex a lot primarily because of their cost and the way the Eldar codex interacts with 5th edition (i.e. basically seen as scoring upgrades). Their Aspect improved statlinetarpitting) but their primary function is providing scoring and anti-infantry and there is little which can be changed about this.
Outside of this, they provide decent anti-infantry firepower and whilst larger squads aren't used as much in Mechdar, they do provide some decently cheap, accurate and not suicidal anti-infantry firepower though dumping extra points into these units for upgrades can make them quite inefficient.
