Not a lawyer, but from a moral perspective, for most justice systems, the punishment is determined after determining whether the person is guilty. On top of that, the prison time for the crime is independent of other crimes (usually). It is almost impossible to balance all the crimes to make it seem fair. How do you even gauge that? Thief < Rapist < Murderer?! Each crime should be taken for what it is rather than what it compares to.
