Does the crime act as evidence that he is a threat to other members of society, or that his presence in wider society puts himself at risk? I mean, if he stole a pack of gum, I wouldn't send him to prison. I'd give him a talking to and try and make him see reason, but not prison. But if it was something like mugging someone for money so that he could go out with his mates, then unless I am sure that he won't do it again I'd send him to prison (if there was enough evidence). Or if he was dealing drugs, or was committing crimes for the rush. You know, if it wasn't a one-time event.
I also guess that if he had a drug addiction (illegal possession), and if the prison he would be sent to wouldn't get him raped and murdered, then I'd rather he spent a couple months in prison getting clean than ruin his life over momentary pleasures.