There's another aspect to incarceration which is just physically keeping people seperated from society so they can't endanger people.
Anyway, it seems obvious to me that all drug use should be decriminalised, as should prositution. The "offenders" in those cases are really the victims.
Usually for people to feel justice has been done, in the case of crimes there does need to be something of a punishment angle, but I think people can be flexible about what exactly constitutes a punishment and some people are way too hard line on it.
"Oh they have a tv in their cell? a SOMEWHAT COMFORTABLE BED??? They deserve to sleep on a pile of jagged glass after what they did to that vending machine"