Assuming that the blank isn't cloned and that for any one blank you can only make one pariah, why the heck would they just be thrown into battle as they are? Obviously a group of four isn't that many but at max per unit and taking only pariahs as your elite choices you can get 30 of these guys! These guys are rare! I've heard of people having a campaign mission of collecting a blank off a planet. If an army of necrons is coming to pick up one guy I sure as hell wouldn't put him or others like him in with the rest of the general army.
It would only make sense to me that a pariah would be an HQ choice. Not only are the pariahs great at taking down enemy psykers because of their aura, but they were once human which would make them more cunning leaders. Further more, pariahs should be named characters because they used to be human. I think an upgrade from a lord to pariah would be a cool thing to do in the new codex, it would be possible to keep the pariahs in the new codex as elite choices by saying that they were weaker blanks whereas the lord was a very strong psychic blank.
I may be totally off base here and if I am please tell me so in the comments, however after reviewing my codex and reading wiki pages a large potion of the lore makes me believe that these blanks are hard to come by and therefore the pariahs would be a more highly valued unit.