Vivi said they are in charged of disposing of the agents who fail. But how'd they deal with someone as strong as Mr. 1 or 2 when Sanji easily took care of them in Little Garden?
It's most likely they're only sent to dispose of lower tier members. Croc did say the only reason Mr. 3 had his position was due to his intelligence, and anyone below at least Mr. 4 is fodder to Sanji at that point anyway.