I think when they are in the normal competition, they are as much on the spot as they are here. They all obviously have their specialist subjects, and a pretty wide general knowledge and can probably make a reasonable guess at most questions - but put them on the spot on a subject they don't know well, and without the usual thinking time, and when it's important to the contestant that they are honest, I can understand their reluctance to give a definitive answer.
I doubt very much that they will do it this way again - the Eggs haven't looked particularly good in it, and it must be demoralising to see that Kevin and Pat/Barry were the firm favourites of all the contestants. It really showed the Eggs (and the producers) which of them the viewers rate as assistants.