Shaw: "Well, if we...if we can't make contact, why did you even bring us here?" Vickers: "Weyland was a superstitious man. He wanted a true believer on board." And with Shaw on board, Holloway HAD to come too - neither he or Shaw would have it any other way. And, until a living Engineer is found and Weyland's ready to step out from behind the curtain, it wouldn't do to have just the two of them going into the temple without some kind of Company-provided aid. It couldn't be just David and the lady doctor - David would likely have to split off from the main party at some point to pursue his "agenda," and the doctor is, well, a doctor. It can't be any of the other main Prometheus crew members, they're needed to fly the ship. So a pair of extra scientists, just to make Shaw/Holloway feel supported and secure. And Doubting Vickers, who doesn't want to see the mission succeed anyway, has no intention of making things any easier for our two "true believers" by hiring any professionals. So she picks out a pair of "anything for a buck" union types.