And surely there is no one who doesn’t know how absurd and even dangerous for society it is to punish theft and murder alike. If the thief realizes that theft by itself carries the same peril as murder, that thought alone will encourage him to kill the victim whom otherwise he would only have robbed. Apart from the fact that he is in no greater danger if he is caught, murder is safer, since he conceals both crimes by killing the witness. Thus while we strive to terrify thieves with extreme cruelty, we really urge them to kill the innocent.
Update 1 (14/6/2010)
Niranjan Rajadhyaksha points to another example of marginal deterrence in the case of punishments for various crimes. If rapists and murderers are punished with the death penalty, what incentive is there for the rapist to leave the victim alive? He would have a strong incentive to kill his victim so as to snuff out all evidence!