[X]Exodite Eldar, followers of the Wildriders of Saim-Hann rebels against the last remnants of the Eldar Empire.
Let's summarize...
The Eldar apparently feel things much stronger than humans, which is one of the main reasons why so many of them drowned in excess before the birth of Slaanesh. If these are the survivors of The Fall, they most likely participated in a lot of sick stuff before they woke up and escaped. I doubt they'd have remained completely untouched, if their entire empire was doing such things. Even if they have, they would have witnessed all of that. Should they be descendants of the actual survivors, and should none of those be alive anymore, they should still have learned a lot, because the escaped Eldar would have tried to keep their new society from making the same mistakes.They are the single best option when it comes to taking care of our Distorted Pleasure Center trait.
If the seers mentioned in the last chapter come from this society, they still have enough knowledge of the art of Divination to retain an entire caste of those. Even if all of the Exodites are the descendants of the escaped Eldar, they are the single best option when it comes to mastering our Dark Visions, or at least preventing them from driving us batshit crazy.
Finally, if these Eldar have retained even a fraction of their race's infamous abilities, any teachings from them will likely focus on agility, speed, evasion and subtlety, rather than overwhelming strength and durability. Something that'd go a long way towards minimazing the negative effects of Uncontrolled Aggression, since it activates when we are injured, and the Eldar focus on avoiding injuries.
All Eldar are psykers, and if we want to master that particular field, they are the ones who can teach us. Even if it's only to make sure we despise Slaanesh too much to ever think of submitting to it. Also, who DOESN'T want to Warp reality to destroy armies without actually lifting a finger, after using Seer Bullshit to manipulate all of your enemies into showing up on a single plain field?
Besides that, there are the other potential benefits: Eldar knowledge(especially considering the Webway), Eldar contacts for potential future alliance(if the Emperor doesn't fuck up and start killing them when he shows up), experience in dealing with the Eldar in general(for future encounters, which may turn out better for us if we reveal that we've been raised by members of their race), and Eldar technology in general.
Mind you, seeing as we've had so many rolls thus far, I'm pretty sure the amount of benefits we'll get will depend on the dice, but the same can be said about ALL of the options we were given.
Also, at this time, the single worst threat to us is Chaos. Slaanesh in particular, probably. Considering the hatred the Eldar have for all things Chaos, and considering how much of that hatred is focused on Slaanesh... Well... They are the most likely option to keep us away from it, or at least help us resist future attempts to corrupt us.