What does a DE allied attachment bring to a Craftworld force?

The most common example I see is The Baron to grant hit and run and stealth to anyone he joins, but are there any other strong synergies?

Does splinterspam or blasterspam bring anything to the CWE that they can't do without their own multishot weapons and fire dragons?

DE can bring the same sort of things as CWE already have but for a lot less points. Blasterborn in a Venom are a lot cheaper than Fire Dragons in a Wave Serpent, for example. They also offer fast assault platforms in the form of their open-topped skimmer transports. These can now be commandeered by their craftworld kin too, so Banshees might occasionally actually see not just the table but also the enemy now.

Splinterspam wounds very high toughness enemies more easily than the strength 6 spam Eldar can bring, but strength 6 spam also has the bonus of being good against light vehicles and all other infantry too. It's also frequently twin-linked. A good volley of splinter fire is a nasty surprise for a daemon prince with Iron Arm, but tends to only do better than scatter lasers in those specific situations. It is on everything already though, so it's not hard to spam it. A couple of Venoms with blasterborn in them could make a great contribution to a CWE army, but we probably need them more than they need us at the moment!

I'd like to add a few things DE provide for the CWE:

-Hit and Run and +1 to begin in form of the duke (looking at you, deathstars)
-poison has already been mentioned, but also cheap super scoring troops in venoms, very fast and with poison support, you don't get a serpent for under 200 pts roughly while a full venom costs much less
-haywire wyches are a lot cheaper than Swooping Hawks, this is big because I've found that my lists would have problems against av14


that's all I can think of from the top of my head right now.

Mr Believer wrote:

DE can bring the same sort of things as CWE already have but for a lot less points. Blasterborn in a Venom are a lot cheaper than Fire Dragons in a Wave Serpent, for example. They also offer fast assault platforms in the form of their open-topped skimmer transports. These can now be commandeered by their craftworld kin too, so Banshees might occasionally actually see not just the table but also the enemy now.

How about taking 3 wracks in a venom then letting 5 fire dragons ride?

Fire Dragons are really expensive, and so often a unit that's relied on to get things done, so I prefer keeping them in a Serpent until they get close enough to deploy out of it and blow something up, rather than stay in a transport and shoot out the top. The Venom can only move 6" if you want them to fire, and with those fusion guns you'll really want to be in melta range. Better that they make it to the tank in the first place too, which they're more likely to do in a Serpent.

They're one unit that, for me, if they were going in a transport (which they pretty much always should), it's a Wave Serpent. Striking Scorpions and Howling Banshees can really benefit from open-topped vehicles for the assault prospects, but I just don't see it being as vital for Fire Dragons. It's difficult enough getting Venoms into range for an assault without them getting shot to bits (although that might be a little easier in 7th, I'm yet to try it), let alone getting a specialist tank hunting unit within 6" of their target in one.

If you only buy one Wave Serpent, buy it for Fire Dragons   

We bring easy kill points.

Lets see. If I was to take Dark Eldar to join Eldar, I'd probably do it for...
The poison. a Raider with 10 warriors with splinter racks dotes out approx 10 wounds a round, in rapid fire. Thats a big deal, IMO.
The jink. While Eldar may jink better, Dark eldar benefit from it more. Most our firepower is coming from INSIDE the transports, which dont have to snap fire because of the jink.
The Haywire. Yay wytches are finally useful again! Also, dont forget. The venom you take them in is also scoring. Drop em off, back up, and hold objectives!

I posted a list elsewhere that uses a Zar/Lilith combo that illustrates how you can make some evil combinations with DE/Eldar. Now my list used DE main and an Eldar allied detachment but you could as easily load up on guardians - field just two Wyche squads and a WK instead of the double Taloi and use Eldar main and a DE detachment. The trick is to flood the table with troops and have a few hammer units. It takes a lot of dakka to remove that many models and it limits how your opponent advances and moves. Flyers have no place to light down. And that point is that DE and Eldar fit seamlessly together in lists.

It is the interactions between the various units and HQs that make DE/Eldar such a strong mix. Lilith/Jain Zar, Beastpack/Shardseer/Karandras, AA from Eldar to supplement our poor AA, etc. Both Eldar and DE can be fielded on their own and there are powerful builds for both but each has units that shores up the other codex weaknesses or enhances a particular unit.