There is no reward for defending the psyker during the side quest to help the psyker close the warp gate but if you let them die you get loot as if you had killed a psyker as well as 5 or 6 marines coming out for you to kill for extra loot.  It would be nice to get the loot only if you helped them and then slowly more powerful waves come out till a pack of champions and not as a failure to close condition.