Everyone in the 40k universe seems to love to use Untouchables as Psyker Assassins. Which makes a certain amount of sense. Psykers are rock and Untouchables are paper.
But the most powerful weapon in the 40k universe is knowledge, and daemonic knowledge has a bit of a storage problem, what with driving people insane and so forth.
So, why not an Untouchable Adept? Knowledge about the Warp can hardly corrupt someone who is immune to its effects. Sure, he can be corrupted the good old fashioned way, but the knowledge is no more inherently corrupting to him than any other valuable secret. Even if he tries to use it, he faces far more barriers than most to bargain with a daemon, since he could not perform or participate in the summoning directly, and the daemon may not even be able to perceive him. And even if he managed to summon and bargain with one, he has less to offer than most mortals (having no soul to give or meaningful worship to offer) and less to gain (can only be granted indirect boons at best).
Seems like a great way to have a character who stocks up on Forbidden Lore and reads all sorts of horrible things without really having to deal with many consequences.