I thought it as well to have Jones with us also. He is not a bad fellow, though an absolute imbecile in his profession. He has one positive virtue. He is as brave as a bulldog and as tenacious as a lobster if he gets his claws upon anyone. Here we are, and they are waiting for us. ("The Red-Headed League")
Sherlock Holmes is talking here about Peter Jones, the Scotland Yard police agent who has been pursuing John Clay for some time. As a general rule, Holmes has very little regard for anyone else's intellect, (with the notable exception of his brother, Mycroft) and Holmes has never had regard for the police force. But in this case Holmes needs someone in addition to Watson to help him nab his criminal, for there will be a gang of men with Clay when the thief emerges from the tunnel. Also, though Sherlock Holmes manages to solve the particulars of the crime entirely unaided, brute force is not his forte. Holmes needs the help of the law to capture John Clay and his gang, and bring them to justice. That he has to use a man whom he considers beneath him in intellect does not bother Holmes; in his mind it is the usual state of affairs!