I think it would somewhat depend on what they're addicted to and other factors. Though I think cannabis is a safer alternative to hard drugs if one is addicted to those, and alcohol if one is addicted to that. Basically cannabis wont kill you so for addicts that have a problem of relapsing a lot it may be in their best interest to have a safer addiction. Or for some it could help relieve withdrawl symptoms making it easier to stop a drug they are addicted to.

Keep in mind people who cant stop smoking are considered 'addicts' and many of them can drink without developing dependency.