Good question. For drug offenders, I don't think we should-- the harsh penalties should go to the dealers. In a hypothetical dream world that would probably never exist but is worth thinking about, drug addicts would have the resources and means to get treatment and turn their lives around without the threat of prison looming over their head.

For other non-violent offenders (people who have stolen or committed white collar crimes or whatever), they need to be disincentivized from causing harm to other people (non-violent harm is still harm). Physical harm isn't the only harm out there, which is why some non-violent things are illegal. When drug-addicts harm people, they should go to prison, too, but the drug addicts who just harm themselves should be sent directly to rehab, not prison.

However, only violent offenders are likely to harm people with guns if they have gun rights, so violent offenders should be stripped of their gun rights. People with felonies but without a history of violence should not be stripped of their gun rights.