The marginalization comes from the fact that you can't participate in society if you are a drug user/abuser. Even if you are healthy enough to be a productive, upstanding member in every other way, the system is designed to tear you down at any moment, based on nothing but the fact that you use an illegal substance. Some people use regularly, but can still maintain and even excel in a job, and keep a healthy social life without anyone even being aware of the drug use or addiction. But they have to live with the knowledge that they are one drug screening away from being out of a job, regardless of performance, and that they can always be arrested the next time they have to purchase their vice, which would likely also lead to loss of a job and social ramifications.

If we treated addicts the same way we treated someone with any other mental or physical ailment, it would make it so much easier for them to actually function in society without "getting clean". Which might actually help a few of the ones who want to get clean, because from what I understand, it is much easier to kick a habit when you have more going for you, like a career and social life.

Just my two cents, from anecdotal evidence knowing drug users in a few different circumstances. Never been a user myself.